Skip to main content

Full text of "Practical talks by an astronomer"

See other formats





Hettrg W. Sage 

^.Z.:^.^ ;2 :i,^. 8/2Zr//.o.. 

6896- I 

The date shows when this volume was taken. 

To renew this book copy the call No. and give to 


copy t 
le librs 

97 SU' 5 SI 5 

' '. :> 

i \m 



All Books subject to 

Books not used for 
instruction or research 
are returnable within 4 

Volumes of periodi 
cals and of pamphlets 
are held in the library 
as much as possible. 
For special purposes 
they are given out for 
a limited time. 

Borrowers should 
not use their library 
privileges for the bene- 
fit of other persons. 

Books not needed 
during recess periods 
should be returned to 
the library, or arrange- 
ments made for their 
return during borrow- 
er's absence, if wanted. 

Books needed by 
more than one person 
are held on the reserve 

B o ok s of special 
value and gift books, 
when the giver wishes 
it, are not allowed to 

Readers are asked to 
report all cases of books 
marked or mutilated. 

Dt> not deface books by marks and writing. 

Cornell University Library 


Practical talks bv an fstronomer 

3 1924 031 322 310 

The original of tliis book is in 
tlie Cornell University Library. 

There are no known copyright restrictions in 
the United States on the use of the text. 

http ://www. arch i ve . o rg/detai Is/cu31 924031 322310 



The Moon. First Quarter. 
Photographed by Loewy and Fuiseux, February 13, 1894. 













Copyright, 1902, by 

Published, April, 190s 





The present volume has not been designed as 
a systematic treatise on astronomy. There are 
many excellent books of that kind, suitable for 
serious students as well as the general reader; 
but they are necessarily somewhat dry and un- 
attractive, because they must aim at complete- 
ness. Completeness means detail, and detail 
means dryness. 

But the science of astronomy contains subjects 
that admit of detached treatment ; and as many 
of these are precisely the ones of greatest general 
interest, it has seemed well to select several, and 
describe them in language free from technicalities. 
It is hoped that the book will thus prove useful 
to persons who do not wish to give the time 
required for a study of astronomy as a whole, but 
who may take pleasure in devoting a half-hour 


now and then to a detached essay on some spe- 
cial topic. 

Preparation of the book in this form has made 
it suitable for prior publication in periodicals ; 
and the several essays have in fact all been 
printed before. But the intention of collecting 
them into a book was kept in mind from the 
first; and while no attempt has been made at 
consecutiveness, it is hoped that nothing of 
merely ephemeral value has been included. 



Navigation at Sea i 

The Pleiades lo 

The Pole-Star i8 

Nebula 27 

Temporary Stars 37 

Galileo 47 

The Planet of 1898 58 

How TO Make a Sun-Dial 69 

Photography in Astronomy 81 

Time Standards of the World 1 1 1 

Motions of the Earth's Pole 131 

Saturn's Rings xM^ 

The Heliometer . . 152 

Occultations 161 

Mounting Great Telescopes 170 

The Astronomer's Pole 184 

The Moon Hoax 199 

The Sun's Destination 210 


The Moon. First Quarter Frontispiece 

Photographed by Loeixiy and Puiseux, February /?, 18^4. 


Spiral Nebula in Constellation Leo 26 

Photographed by Keeler, February 34, /goo. 

Nebula in Andromeda 28 

Photographed by Barnard, November si, iSq2. 

The ''Dumb-Bell" Nebula 34 

Photographed by Keeler, July 31, i8qq. 

Star-Field in Constellation Monoceros .... 84 

Photographed by Barnard, February i, i8g4. 

Solar Corona. Total Eclipse 108 

Photographed by Campbell, January 32, i8qS ; Jeur, India. 

Forty-Inch Telescope, Yerkes Observatory . . .170 
Yerkes Observatory, University of Chicago . , .176 



A SHORT time ago the writer had occasion to 
rummage among the archives of the Royal As- 
tronomical Society in London, to consult, if pos- 
sible, the original manuscripts left by one Stephen 
Groombridge, an English astronomer of the good 
old day^s, who died in 1832. It was known that 
they had been filed away about a generation ago, 
by the late Sir George Airy, who was Astronomer 
Royal of England between the years 1835 ^^^ 
1 88 1. After a long search, a large and dusty 
box was found and opened. It was filled with 
documents, of which the topmost was in Sir 
George's own handwriting, and began substan- 
tially as follows : 

" List of articles within this box. 
"No. I, This list. 
No. 2, etc., etc." 



Astronomical precision can no further go : he 
had listed even the list itself. Truly, Airy was 
rightly styled " prince of precisians." A worthy 
Astronomer Royal was he, to act under the 
royal warrant of Charles II., who established the 
office in 1675. Down to this present day that 
warrant still makes it the duty of His Majesty's 
Astronomer " to apply himself with the most 
exact care and diligence to the rectifying of the 
tables of the motions of the heavens and the 
places of the fixed stars, in order to find out the 
so much desired longitude at sea, for the per- 
fecting the art of navigation." 

The " so much desired longitude at sea " is, 
indeed, a vastly important thing to a maritime 
nation like England. And only in comparatively 
recent years has it become possible and easy for 
vessels to be navigated with safety and conven- 
ience upon long voyages. The writer was well 
acquainted with an old sea-captain of New York, 
who had commanded one of the earliest transatlan- 
tic steamers, and who died only a few years ago. 
He had a goodly store of ocean yarn, fit and 
ready for the spinning, if he could but find some- 


one who, like himself, had known and loved the 
ocean. In his early sea-going days, only the 
wealthiest of captains owned chronometers. This 
instrument is now considered indispensable in 
navigation, but at that time it was a new in- 
vention, very rare and costly. Upon a certain 
voyage from England to Rio Janeiro, in South 
America, the old captain could remember the 
following odd method of navigation : The ship 
was steered by compass to the southward and 
westward, more or less, until the skipper's 
antique quadrant showed that they had about 
reached the latitude of Rio. Then they swung 
her on a course due west by compass, and away 
she went for Rio, relying on the lookout man for- 
ward to keep the ship from running ashore. For 
after a certain lapse of time, being ignorant of the 
longitude, they could not know whether they 
would " raise " the land within an hour or in six 
weeks. We are glad of an opportunity to put 
this story on record, for the time is not far distant 
when there will be no man left among the living 
who can remember how ships were taken across 
the seas in the good old days before chronometers. 



Anyone who has ever been a passenger on a 
great transatlantic liner of to-day knows what an 
important, imposing personage is the brass-bound 
skipper. A very different creature is he on the 
deck of his ship from the modest seafaring man we 
meet on land, clad for the time being in his shore- 
going togs. But the captain's dignity is not all 
brass buttons and gold braid. He has behind 
him the powerful support of a deep, delightful 
mystery. He it is who " takes the sun " at 
noon, and finds out the ship's path at sea. And 
in truth, regarded merely as a scientific experi- 
ment, the guiding of a vessel across the unmarked 
trackless ocean has few equals within the whole 
range of human knowledge. It is our purpose 
here to explain quite briefly the manner in which 
this seeming impossibility is accomplished. We 
shall not be able to go sufficiently into details to 
enable him who reads to run and navigate a mag- 
nificent steamer. But we hope to diminish some- 
what that small part of the captain's vast dignity 
which depends upon his mysterious operations 
with the sextant. 

To begin, then, with the sextant itself. It is 



nothing but an instrument with which we can 
measure how high up the sun is in the sky. Now, 
everyone knows that the sun slowly climbs the 
sky in the morning, reaches its greatest height at 
noon, and then slowly sinks again in the after- 
noon. The captain simply begins to watch the 
sun through the sextant shortly before noon, 
and keeps at it until he discovers that the sun is 
just beginning to descend. That is the instant of 
noon on the ship. The captain quickly glances 
at the chronometer, or calls out " noon " to an 
officer who is near that instrument. And so the 
error of the chronometer becomes known then 
and there without any further astronomical cal- 
culations whatever. Navigators can also find the 
chronometer error by sextant observations when 
the sun is a long way from noon. The methods 
of doing this are somewhat less simple than for 
the noon observation ; they belong to the details 
of navigation, into which we cannot enter here. 

Incidentally, the captain also notes with the 
sextant how high the sun was in the sky at the 
noon observation. He has in his mysterious 
" chart-room " some printed astronomical tables, 



which tell him in what terrestrial latitude the sun 
will have precisely that height on that particular 
day of the year. Thus the terrestrial latitude be- 
comes known easily enough, and if only the cap- 
tain could get his longitude too, he would know 
just where his ship was that day at noon. 

We have seen that the sextant observations 
furnish the error of the chronometer according to 
ship's time. In other words, the captain is in 
possession of the correct local time in the place 
where the ship actually is. Now, if he also had 
the correct time at that moment of some well- 
known place on shore, he would know the differ- 
ence in time between that place on shore and the 
ship. But every traveller by land or sea is aware 
that there are always differences of time between 
different places on the earth. If a watch be 
right on leaving New York, for instance, it will 
be much too fast on arriving at Chicago or San 
Francisco ; the farther you go the larger becomes 
the error of your watch. In fact, if you could 
find out how much your watch had gone into 
error, you would in a sense know how far east or 
west you had travelled. 



^ow the captain's chronometer is set to cor- 
rect " Greenwich time " on shore before the ship 
leaves port. His observations having then told 
him how much this is wrong on that particular 
day, and in that particular spot where the ship is, 
he knows at once just how far he has travelled 
east or west from Greenwich. In other words, 
he knows his "longitude from Greenwich," for 
longitude is nothing more than distance from 
Greenwich in an east-and-west direction, just as 
latitude is only distance from the equator meas- 
ured in a north-and-south direction. Greenwich 
observatory is usually selected as the beginning 
of things for measuring longitudes, because it is 
almost the oldest of existing astronomical estab- 
lishments, and belongs to the most prominent 
maritime nation, England. 

One of the most interesting bits of astronomi- 
cal history was enacted in connection with this 
matter of longitude. From what has been said, 
it is clear that the ship's longitude will be ob- 
tained correctly only if the chronometer has kept 
exact time since the departure of the ship from 
port. Even a very small error of the chronom- 


eter will throw out the longitude a good many 
miles, and we can understand readily that it must 
be difficult in the extreme to construct a mechan- 
ical contrivance capable of keeping exact time 
when subjected to the rolling and pitching of a 
vessel at sea. 

It was as recently as the year 1736 that the 
first instrument capable of keeping anything like 
accurate time at sea was successfully completed. 
It was the work of an English watchmaker 
named John Harrison, and is one of the few 
great improvements In matters scientific which 
the world owes to a desire for winning a money 
prize. It appears that in 17 14 a committee was 
appointed by the House of Commons, with no 
less a person than Sir Isaac Newton himself as 
one of its members, to consider the desirability 
of offering governmental encouragement for the 
invention of some means of finding the longitude 
at sea. Finally, the British Government offered 
a reward of ^50,000 for an instrument which 
would find the longitude within sixty miles ; 
^75,cx)0, if within forty miles, and ^100,000, if 
within thirty miles. Harrison's chronometer was 



finished in 1736, but he did not receive the final 
payment of his prize until 1764. 

We shall not enter into a detailed account of 
the vexatious delays and official procedures to 
which he was forced to submit during those 
twenty-eight long years. It is a matter of satis- 
faction that Harrison lived to receive the money 
which he had earned. He had the genius to 
plan and master intricate mechanical details, but 
perhaps he lacked in some degree the ability of 
tongue and pen to bring them home to others. 
This may be the reason he is so little known, 
though it was his fortune to contribute so large 
and essential a part to the perfection of modern 
navigation. Let us hope this brief mention may 
serve to recall his memory from oblivion even for 
a fleeting moment ; that we may not have written 
in vain of that longitude to which his life was 


Famed in legend ; sung by early minstrels of 
Persia and Hindustan ; 

" — like a swarm of fire-flies tangled in a silver braid " ; 

yonder distant misty little cloud of Pleiades has 
always won and held the imagination of men. 
But it was not only for the inspiration of poets, 
for quickening fancy into song, that the seven 
daughters of Atlas were fixed upon the firma- 
ment. The problems presented by this group of 
stars to the unobtrusive scientific investigator are 
among the most interesting known to astronomy. 
Their solution is still very incomplete, but what 
we have already learned may be counted justly 
among the richest spoils brought back by sci- 
ence from the stored treasure-house of Nature's 

The true student of astronomy is animated by 
no mere vulgar curiosity to pry into things hid- 
den. If he seeks the concealed springs that 


move the complex visible mechanism of the 
heavens, he does so because his imagination is 
roused by the grandeur of what he sees ; and 
deep down within him stirs the true love of the 
artist for his art. For it is indeed a fine art, that 
science of astronomy. 

It can have been no mere chance that has 
massed the Pleiades from among their fellow 
stars. Men of ordinary eyesight see but a half- 
dozen distinct objects in the cluster ; those of 
acuter vision can count fourteen ; but it is not 
until we apply the space-penetrating power of 
the telescope that we realize the extraordinary 
scale upon which the system of the Pleiades is 
constructed. With the Paris instrument Wolf in 
1876 catalogued 625 stars in the group; and the 
searching photographic survey of Henry in 1887 
revealed no less than 2,326 distinct stars within 
and near the filmy gauze of nebulous matter al- 
ways so conspicuous a feature of the Pleiades. 

The means at our disposal for the study of 
stellar distances are but feeble. Only in the case 
of a very small number of stars have we been 
able to obtain even so much as an approximate 


estimate of distance. The most powerful obser- 
vational machinery, though directed by the tried 
skill of experience, has not sufficed to sound the 
profounder depths of space. The Pleiad stars 
are among those for which no measurement of 
distance has yet been made, so that we do not 
know whether they are all equally far away from 
us. We see them projected on the dark back- 
ground of the celestial vault ; but we cannot tell 
from actual measurement whether they are all 
situated near the same point in space. It may be 
that some are immeasurably closer to us than are 
the great mass of their companions ; possibly we 
look through the cluster at others far behind it, 
clinging, as it were, to the very fringe of the visi- 
ble universe. 

Farther on we shall find evidence that some- 
thing like this really is the case. But under no 
circumstances is it reasonable to suppose that the 
whole body of stars can be strung out at all sorts 
of distances near a straight line pointing in the 
direction of the visible cluster. Such a distribu- 
tion may perhaps remain among the possibilities, 
so long as we cannot measure directly the actual 


distances of the individual stars. But science 
never accepts a mere possibility against which we 
can marshal strong circumstantial evidence. We 
may conclude on general principles that the 
gathering of these many objects into a single 
close assemblage denotes community of origin 
and interests. 

The Pleiades then really belong to one an- 
other. What is the nature of their mutual tie ? 
What is their mystery, and can we solve it? 
The most obvious theory is, of course, suggested 
by what we know to be true within our own 
solar system. We owe to Newton the beautiful 
conception of gravitation, that unique law by 
means of which astronomers have been en- 
abled to reduce to perfect order the seeming 
tangle of planetary evolutions. The law really 
amounts, in effect, to this : All objects suspended 
within the vacancy of space attract or pull one 
another. How they can do this without a visi- 
ble connecting link between them is a mystery 
which may always remain unsolved. But mys- 
tery as it is, we must accept it as an ascertained 
fact. It is this pull of gravitation that holds to- 



gether the sun and planets, forcing them all to 
follow out their due and proper paths, and so to 
continue throughout an unbroken cycle until the 
great survivor. Time, shall be no more. 

This same gravitational attraction must be at 
work among the Pleiades. They, too, like our- 
selves, must have bounds and orbits set and 
interwoven, revolutions and gyrations far more 
complex than the solar system knows. The 
visual discovery of such motion of rotation 
among the Pleiades may be called one of the 
pressing problems of astronomy to-day. We 
feel sure that the time is ripe, and that the dis- 
covery is actually being made at the present mo- 
ment : for a generation of men Is not too great a 
period to call a moment, when we have to deal 
with cosmic time. 

It is indeed the lack of observations extending 
through sufficient centuries that stays our hand 
from grasping the coveted result. The Pleiades 
are so far from us that we cannot be sure of 
changes among them. Magnitudes are always 
relative. It matters not how large the actual 
movements may be ; if they are extremely small 



in comparison with our distance, they must 
shrink to nothingness in our eyes. Trembling 
on the verge of invisibility, elusive, they are in 
that borderland where science as yet but feels her 
way, though certain that the way is there. 

The foundations of exact modern knowledge 
of the group were laid by Bessel about 1840. 
With the modesty characteristic of the great, he 
says quite simply that he has made a number of 
measures of the Pleiades, thinking that the time 
may come when astronomers will be able to find 
some evidence of motion. In this unassuming 
way he prefaces what is still the classic model of 
precision and thoroughness in work of this kind. 
Bessel cleared the ground for a study of inter- 
stellar motion within the close star-clusters ; and 
it is probable that only by such study may we 
hope to demonstrate the universality of the law 
of gravitation in cosmic space. 

Bessel's acuteness in forecasting the direction 
of coming research was amply verified by the 
work of Elkin in 1885 at Yale College. Pro- 
vided with a more modern instrument, but sim- 
ilar to Bessel's, Elkin was able to repeat his 



observations with a slight increase of precision. 
Motions in the interval of forty-five years, suffi- 
ciently great to hint at coming possibilities, were 
shown conclusively to exist. Six stars at all 
events have been fairly excluded from the group 
on account of their peculiar motions shown by 
Elkin's research. It is possible that they are 
merely seen in the background through the in- 
terstices of the cluster itself, or they may be sus- 
pended between us and the Pleiades, in either 
case having no real connection with the group. 
Finally, these observations make it reasonably 
certain that many of the remaining mass of stars 
really constitute a unit aggregation in space. 
Astronomers of a coming generation will again 
repeat the Besselian work. At present we have 
been able to use his method only for the separa- 
tion from the true Pleiades of chance stars that 
happen to lie in the same direction. Let us hope 
that man shall exist long enough upon this earth 
to see the clustered stars themselves begin and 
carry out such gyrations as gravitation imposes. 

These will doubtless be of a kind not even 
suggested by the lesser complexities of our solar 



system. For the most wonderful thing of all 
about the Pleiades seems to point to an intricacy 
of structure whose details may be destined to 
shake the confidence of the profoundest mathe- 
matician. There is an extraordinary nebulous 
condensation that seems to pervade the entire 
space occupied by the stellar constituents of the 
group. The stars are swimming in a veritable 
sea of luminous cloud. There are filmy tenuous 
places, and again condensing whirls of material 
doubtless still in the gaseous or plastic stage. 
Most noticeable of all are certain almost straight 
lines of nebula that connect series of stars. In 
one case, shown upon a photograph made by 
Henry at Paris, six stars are strung out upon 
such a hazy line. We might give play to fancy, 
and see in this the result of some vast eruption 
of gaseous matter that has already begun to 
solidify here and there into stellar nuclei. But 
sound science gives not too great freedom to 
mere speculative theories. Her duty has been 
found in quiet research, and her greatest rewards 
have flowed from imaginative speculation, only 
when tempered by pure reason, 



One of the most brilliant observations of the 
last few years is Campbell's recent discovery of 
the triple character of this star. Centuries and 
centuries ago, when astronomy, that venerable 
ancient among the sciences, was but an infant, 
the pole-star must have been considered the very 
oldest of observed heavenly bodies. In the be- 
ginning it was the only sure guide of the naviga- 
tor at night, just as to this day it is the founda- 
tion-stone for all observational stellar astronomy 
of precision. There has never been a time in 
the history of astronomy when the pole-star 
might not have been called the most frequently 
measured object in the sky of night. So it is 
indeed strange that we should find out some- 
thing altogether new about it after all these ages 
of study. 

But the importance of the discovery rests 
upon a surer foundation than this. The method 



by which it has been made is almost a new one 
in the science. A generation ago, men thought 
the " perfect science," for so we love to call 
astronomy, could advance only by increasing a 
little the exact precision of observation. The 
citadel of perfect truth might be more closely in- 
vested ; the forces of science might push forward 
step by step ; the machinery of research might 
be strengthened, but that a new engine of inves- 
tigation would be discovered capable of penetrat- 
ing where no telescope can ever reach, this, in- 
deed, seemed far beyond the liveliest hope of 
science. Even the discoverer of the spectro- 
scope could never have dreamed of its possibil- 
ities, could never have foreseen its successes, its 

The very name of this instrument suggests 
mystery to the popular mind. It is set down at 
once among the things too difficult, too intricate, 
too abstruse to understand. Yet in its essentials 
there is nothing about the spectroscope that can- 
not be made clear in a few words. Even the 
modern " undulatory theory " of light itself is 
terrible only in the length of its name. Any- 



one who has seen the waves of ocean roll, roll, 
and ever again roll in upon the shore, can form a 
very good notion of how light moves. 'Tis just 
such a series of rolling waves ; started perhaps 
from some brilliant constellation far out upon the 
confining bounds of the visible universe, or per- 
haps coming from a humble light upon the stu- 
dent's table ; yet it is never anything but a 
succession of rolling waves. Only, unlike the 
waves of the sea, light waves are all excessively 
small. We should call one whose length was a 
twenty-thousandth of an inch a big one ! 

Now the human eye possesses the property of 
receiving and understanding these little waves. 
The process is an unconscious one. Let but a 
set of these tiny waves roll up, as it were, out of 
the vast ocean of space and impinge upon the eye, 
and all the phenomena of light and color become 
what we call " visible." We see the light. 

And how does all this find an application in 
astronomy ? Not to enter too much into technical 
details, we may say that the spectroscope is an in- 
strument which enables us to measure the length 
of these light waves, though their length is so 



exceedingly small. The day has indeed gone by 
when that which poets love to call the Book of 
Nature was printed in type that could be read 
by the eye unaided. Telescope, microscope, and 
spectroscope are essential now to him who would 
penetrate any of Nature's secrets. But measure- 
ments with a telescope, like eye observations, are 
limited strictly to determining the directions in 
which we see the heavenly bodies. Ever since 
the beginning of things, when old Hipparchus and 
Ulugh Beg made the first rude but successful at- 
tempts to catalogue the stars, the eye and telescope 
have been able to measure only such directions. 
We aim the telescope at a star, and record the 
direction in which it was pointed. Distances in 
astronomy can never be measured directly. All 
that we know of them has been obtained by cal- 
culations based upon the Newtonian law of gravi- 
tation and observations of directions. 

Now the spectroscope seems to offer a sort of 
exception to this rule. Suppose we can measure 
the wave-lengths of the light sent us from a star. 
Suppose again that the star is itself moving swiftly 
toward us through space, while continually set- 


ting in motion the waves of light that are ulti- 
mately to reach the waiting astronomer. Evi- 
dently the light waves will be crowded together 
somewhat on account of the star's motion. More 
waves per second will reach us than would be re- 
ceived from a star at rest. It is as though the 
light waves were compressed or shortened a little. 
And if the star is leaving us, instead of coming 
nearer, opposite effects will occur. We have then 
but to compare spectroscopically starlight with 
some artificial source of light in the observatory in 
order to find out whether the star is approaching 
us or receding from us. And by a simple process 
of calculation this stellar motion can be obtained 
in miles per second. Thus we can now actually 
measure directly, in a certain sense, linear speed 
in stellar space, though we are still without the 
means of getting directly at stellar distances. 

But the most wonderful thing of all about these 
spectroscopic measures is the fact that it makes 
no difference whatever how far away is the star 
under observation. What we learn through the 
spectroscope comes from a study of the waves 
themselves, and it is of no consequence how far 



they have travelled, or how long they have been 
a-coming. For it must not be supposed that 
these waves consume no time in passing from a 
distant star to our own solar system. It is true 
that they move exceeding fast ; certainly 1 80,000 
miles per second may be called rapid motion. 
But if this cosmic velocity of light is tremendous, 
so also are cosmic distances correspondingly vast. 
Light needs to move quickly coming from a star, 
for even at the rate of motion we have mentioned 
it requires many years to reach us from some of 
the more distant constellations. It has been well 
said that an observer on some far-away star, if 
endowed with the power to see at any distance, 
however great, might at this moment be looking 
on the Crusaders proceeding from Europe against 
the Saracen at Jerusalem. For it is quite pos- 
sible that not until now has the light which would 
make the earth visible had time to reach him. 
Yet distant as such an observer might be, light 
from the star on which he stood could be meas- 
ured in the spectroscope, and would infallibly tell 
us whether the earth and star are approaching in 
space or gradually drawing farther asunder. 



The pole-star is not one of the more distant 
stellar systems. We do not know how far it is 
from us very exactly, but certainly not less than 
forty or fifty years are necessary for its light to 
reach us. The star might have gone out of ex- 
istence twenty years ago, and we not yet know 
of it, for we would still be receiving the light 
which began its long journey to us about 1850 or 
i860. But no matter what may be its distance, 
Campbell found by carefiil observations, made in 
the latter part of 1 896, that the pole-star was then 
approaching the earth at the rate of about twelve 
miles per second. So far there was nothing espe- 
cially remarkable. But in August and September 
of the present year twenty-six careful determina- 
tions were made, and these showed that now the 
rate of approach varied between about five and 
nine miles per second. More astonishing still, 
there was a uniform period in the changes of 
velocity. In about four days the rate of motion 
changed from about five to nine miles and back 
again. And this variation kept on with great 
regularity. Every successive period of four days 
saw a complete cycle of velocity change forward 



and back between the same limits. There can be 
but one reasonable explanation. This star must 
be a double, or " binary " star. The two com- 
ponents, under the influence of powerful mutual 
gravitational attraction, must be revolving in a 
mighty orbit. Yet this vast orbit, as a whole, with 
the two great stars in it, must be approaching our 
part of the universe all the time. For the spectro- 
scope shows the velocity of approach to increase 
and diminish, indeed, but it is always present. 
Here, then, is this great stellar system, having a 
four-day revolution of its own, and yet swinging 
rapidly through space in our direction. Nor is 
this all. One of the component stars must be 
nearly or quite dark ; else its presence would in- 
fallibly be detected by our instruments. 

And now we come to the most astonishing 
thing of all. How comes it that the average 
rate of approach of the " four-day system," as a 
whole, changed between 1896 and 1899? In 
1896 only this velocity of the whole system was 
determined, the four-day period remaining undis- 
covered until the more numerous observations of 
1899. But even without considering the four- 



day period, the changing velocity of the entire 
system offers one of those problems that exact 
science can treat only by the help of the imagina- 
tion. There must be some other great centre of 
attraction, some cosmic giant, holding the visible 
double pole-star under its control. Thus, that 
which we see, and call the pole-star, is in reality 
threading its path about the third and greatest 
member of the system, itself situated in space, we 
know not where. 


Spiral Nebula in Constellation Leo. 

Photographed by Keeler, February 24, 1900. 

Exposure, three hours, fifty minutes. 


Scattered about here and there among the 
stars are certain patches of faint luminosity called 
by astronomers Nebulae. These " little clouds " 
of filmy light are among the most fascinating of 
all the kaleidoscopic phenomena of the heavens ; 
for it needs but a glance at one of them to give 
the impression that here before us is the stuff of 
which worlds are made. All our knowledge of 
Nature leads us to expect in her finished work 
the result of a series of gradual processes of de- 
velopment. Highly organized phenomena such 
as those existing in our solar system did not 
spring into perfection in an instant. Influential 
forces, easy to imagine, but difficult to define, 
must have directed the slow, sure transformation 
of elemental matter into sun and planets, things 
and men. Therefore a study of those forces 
and of their probable action upon nebular 
material has always exerted a strong attraction 



upon the acutest thinkers among men of exact 

Our knowledge of the nebulae is of two kinds — 
that which has been ascertained from observation 
as to their appearance, size, distribution, and dis- 
tance ; and that which is based upon hypotheses 
and theoretical reasoning about the condensation 
of stellar systems out of nebular masses. It so 
happens that our observational material has re- 
ceived a very important addition quite recently 
through the application of photography to the 
delineation of nebulae, and this we shall describe 
farther on. 

Two nebulae only are visible to the unaided 
eye. The brighter of these is in the constellation 
Andromeda ; it is of oval or elliptical shape, and 
has a distinct central condensation or nucleus. 
Upon a photograph by Roberts it appears to 
have several concentric rings surrounding the 
nebula proper, and gives the general impression 
of a flat round disk foreshortened into an oval 
shape on account of the observer's position not 
being square to the surface of the disk. Very 

recent photographs of this nebula, made with the 


Nebula in Andromeda. 

Lower object in tlie photograph is a Comet. 

Photographed by Barnard, November 21, 1892. 


three-foot reflecting telescope of the Lick Ob- 
servatory, bring out the fact that it is really spiral 
in form, and that the outlying nebulous rings are 
only parts of the spires in a great cosmic whorl. 

This Andromeda nebula is the one in which 
the temporary star of 1885 appeared. It blazed 
up quite suddenly near the apparent centre of the 
nebula, and continued in view for six months, 
fading finally beyond the reach of our most 
powerful telescopes. There can be little doubt 
that the star was actually in the nebula, and not 
merely seen through it, though in reality situated 
in the extreme outlying part of space at a distance 
immeasurably greater than that separating us from 
the nebula itself. Such an accidental superpo- 
sition of nebula and star might even be due to 
sudden incandescence of a new star between us 
and the nebula. In such a case we should see 
the star projected upon the surface of the nebula, 
so that the superposition would be identical with 
that actually observed. Therefore, while it is, 
indeed, possible that the star may have been either 
far behind the nebula or in front of it, we must 
accept as more probable the supposition that 



there was a real connection between the two. In 
that case there is little doubt that we have actu- 
ally observed one of those cataclysms that mark 
successive steps of cosmic evolution. We have 
no thoroughly satisfactory theory to account for 
such an explosive catastrophe within the body of 
the nebula itself. 

The other naked-eye nebula is in the constella- 
tion Orion. In the telescope it is a more strik- 
ing object, perhaps, than the Andromeda nebula ; 
for it has no well-defined geometrical form, but 
consists of an immense odd-shaped mass of light 
enclosing and surrounding a number of stars. It 
is unquestionably of a very complicated structure, 
and is, therefore, less easily studied and explained 
than the nebulae of simpler form. There is no 
doubt that the Orion nebula is composed of lumi- 
nous gas, and is not merely a cluster of small 
stars too numerous and too near together to be 
separated from each other, even in our most 
powerful telescopes. It was, indeed, supposed, 
until about forty years ago, that all the nebulae 
are simply irresolvable star-clusters ; but we now 

have indisputable evidence, derived from the 



spectroscope, that many nebulas are composed of 
true gases, similar to those with which we experi- 
ment in chemical laboratories. This spectro- 
scopic proof of the gaseous character of nebulae is 
one of the most important discoveries contrib- 
uted by that instrument to our small stock of 
facts concerning the structure of the sidereal uni- 

Coming now to the smaller nebulae, we find a 
great diversity of form and appearance. Some 
are ring-shaped, perhaps having a less brilliant 
nebulosity within the ring. Many show a central 
condensation of disk-like appearance (planetary 
nebulae), or have simply a star at the centre 
(nebulous stars). Altogether about ten thousand 
such objects have been catalogued by successive 
generations of astronomers since the invention of 
the telescope, and most of these have been re- 
ported as oval in form. Now we have already 
referred to the important addition to our knowl- 
edge of the nebulae obtained by recent photo- 
graphic observations ; and this addition consists in 
the discovery that most of these oval nebulae are 
in reality spirals. Indeed, it appears that the spiral 



type is the normal type, and that nebulae of irreg- 
ular or other forms are exceptions to the gen- 
eral rule. Even the great Andromeda nebula, as 
we have seen, is now recognized as a spiral. 

The instrument with which its convolute 
structure was discovered is a three-foot reflecting 
telescope, made by Common of England, and now 
mounted at the Lick Observatory, in California. 
The late Professor Keeler devoted much of his 
time to photographing nebulae during the last year 
or two. He was able to establish the important 
fact just mentioned, that most nebulae formerly 
thought to be mere ovals, turn out to be spiral 
when brought under the more searching scrutiny 
of the photographic plate applied at the focus of 
a telescope of great size, and with an exposure to 
the feeble nebular light extending through three 
or four consecutive hours. 

Many of the spirals have more than a single 
volute. It is as though one were to attach a 
number of very flexible rods to an axle, like 
spokes of a wheel without a rim and then revolve 
the axle rapidly. The flexible rods would bend 
under the rapid rotation, and form a series of 



spiral curves not unlike many of these nebulae. 
Indeed, it is impossible to escape the conviction 
that these great celestial whorls are whirling 
around an axis. And it is most important in 
the study of the growth of worlds, to recognize 
that the type specimen is a revolving spiral. 
Therefore, the rotating flattened globe of incan- 
descent matter postulated by Laplace's nebular 
hypothesis would make of our solar system an 
exceptional world, and not a type of stellar evo- 
lution in general. 

Keeler's photographs have taught us one thing 
more. Scarcely is there a single one of his nega- 
tives that does not show nebulae previously un- 
catalogued. It is estimated that if this process of 
photography could be extended so as to cover the 
entire sky, the whole number of nebulae would 
add up to the stupendous total of 120,000; and 
of these the great majority would be spiral. 

When we approach the question of the distri- 
bution of nebulae in different parts of the sky, as 
shown by their catalogued positions, we are met 
by a curious fact. It appears that the region in 
the neighborhood of the Milky Way is espe- 



cially poor in nebulae, whereas these objects seem 
to cluster in much larger numbers about those 
points in the sky that are farthest from the 
Milky Way. But we know that the Milky 
Way is richer in stars than any other part of the 
sky, since it is, in fact, made up of stellar bodies 
clustered so closely that it is wellnigh impossible 
to see between them in the denser portions. 
Now, it cannot be the result of chance that the 
stars should tend to congregate in the Milky 
Way, while the nebulae tend to seek a position as 
far from it as possible. Whatever may be the 
cause, we must conclude that the sidereal system, 
as we see it, is in general constructed upon a sin- 
gle plan, and does not consist of a series of uni- 
verses scattered at random throughout space. If 
we are to suppose that nebula turn into stars as 
a result of condensation or any other change, 
then it is not astonishing to find a minimum of 
nebulae where there is a maximum of stars, since 
the nebulae will have been consumed, as it were, 
in the formation of the stars. 

It is never advisable to push philosophical 
speculation very far when supported by too slen- 


The "Damb-Bell" Nebula. 

Photographed by Keeler, July 31, i8c 
Exposure, three hours. 


der a basis of fact. But if we are to regard the 
visible universe as made up on the whole of a 
single system of bodies, we may well ask one or 
two questions to be answered by speculative the- 
ory. We have said the stars are not uniformly 
distributed in space. Their concentration in the 
Milky Way, forming a narrow band dividing the 
sky into two very nearly equal parts, must be 
due to their being actually massed in a thin disk 
or ring of space within which our solar system is 
also situated. This thin disk projected upon the 
sky would then appear as the narrow star-band 
of the Milky Way. Now, suppose this disk has 
an axis perpendicular to itself, and let us imagine 
a rotation of the whole sidereal system about 
that axis. Then the fact that the visible nebu- 
lae are congregated far from the Milky Way 
means that they are actually near the imaginary 

Possibly the diminished velocity of motion 
near the axis may have something to do with 
the presence of the nebulae there. Possibly the 
nebulae themselves have axes perpendicular to 
the plane of the Milky Way. If so, we should 



see the spiral nebulae near the Milky Way edge- 
wise, and those far from it without foreshort- 
ening. Thus, the paucity of nebulas near the 
Milky Way may be due in part to the increased 
difficulty of seeing them when looked at edge- 
wise. Indeed, there is no limit to the possibil- 
ities of hypothetical reasoning about the nebular 
structure of our universe ; unfortunately, the 
whole question must be placed for the present 
among those intensely interesting cosmic prob- 
lems awaiting elucidation, let us hope, in this 
new century. 



Nothing can be more erroneous than to sup- 
pose that the stellar multitude has continued un- 
changed throughout all generations of men. 
" Eternal fires " poets have called the stars ; yet 
they burn like any little conflagration on the 
earth ; now flashing with energy, brilliant, incan- 
descent, and again sinking into the dull glow of 
smouldering half-burned ashes. It is even prob- 
able that space contains many darkened orbs, stars 
that may have risen in constellations to adorn 
the skies of prehistoric time — now cold, unseen, 
unknown. So far from dealing with an un- 
varying universe, it is safe to say that sidereal 
astronomy can advance only by the discovery of 
change. Observational science watches with un- 
tiring industry, and night hides few celestial events 
from the ardent scrutiny of astronomers. Old 
theories are tested and newer ones often perfected 
by the detection of some slight and previously 



unsuspected alteration upon the face of the sky. 
The interpretation of such changes is the most 
difficult task of science ; it has taxed the acutest 
intellects among men throughout all time. 

If, then, changes can be seen among the stars, 
what are we to think of the most important change 
of all, the blazing into life of a new stellar system ? 
Fifteen times since men began to write their 
records of the skies has the birth of a star been 
seen. Surely we may use this term when we 
speak of the sudden appearance of a brilliant lu- 
minary where nothing visible existed before. But 
we shall see further on that scientific considera- 
tions make it highly probable that the phenom- 
enon in question does not really involve the crea- 
tion of new matter. It is old material becoming 
suddenly luminous for some hidden reason. In 
fact, whenever a new object of great brilliancy has 
been discovered, it has been found to lose its light 
again quite soon, ending either in total extinction 
or at least in comparative darkness. It is for this 
reason that the name " temporary star " has been 
applied to cases of this kind. 

The first authenticated instance dates from the 


year 134 b.c, when a new star appeared in the 
constellation Scorpio. It was this star that led 
Hipparchus to construct his stellar catalogue, the 
first ever made. It occurred to him, of course, 
that there could be but one way to make sure in 
the future that any given object discovered in the 
sky was new ; it was necessary to make a com- 
plete list of everything visible in his day. Later 
astronomers need then only compare Hippar- 
chus's catalogue with the heavens from time to 
time in order to find out whether anything un- 
known had appeared. This work of Hipparchus 
became the foundation of sidereal study, and led 
to most important discoveries of various kinds. 

But no records remain concerning his new star 
except the bare fact of its appearance in Scorpio. 
Hipparchus's published works are all lost. We 
do not even know the exact place of his birth, 
and as for those two dates of entry and exit that 
history attaches to great names — we have them 
not. Yet he was easily the first astronomer of 
antiquity, one of the first of all time ; and we 
know of him only from the writings of Ptolemy, 
who lived three hundred years after him. 



More than five centuries elapsed before another 
temporary star was entered in the records of 
astronomy. This happened in the year 389 a,d., 
when a star appeared in Aquila ; and of this one 
also we know nothing further. But about twelve 
centuries later, in November, 1572, a new and 
brilliant object was found in the constellation 
Cassiopeia. It is known as Tycho's star, since it 
was the means of winning for astronomy a man 
who will always take high rank in her annals, 
Tycho Brahe, of Denmark. When he first saw 
this star, it was already very bright, equalling even 
Venus at her best ; and he continued a careful 
series of observations for sixteen months, when it 
faded finally from his view. The position of the 
new star was measured with reference to other stars 
in the constellation Cassiopeia, and the results of 
Tycho's observations were finally published by 
him in the year 1573. It appears that much urg- 
ing on the part of friends was necessary to induce 
him to consent to this publication, not because of 
a modest reluctance to rush into print, but for the 
reason that he considered it undignified for a no- 
bleman of Denmark to be the author of a book ! 



An important question in cosmic astronomy is 
opened by Tycho's star. Did it really disappear 
from the heavens when he saw it no more, or had 
its lustre simply been reduced below the visual 
power of the unaided eye ? Unfortunately, 
Tycho's observations of the star's position in the 
constellation were necessarily crude. He pos- 
sessed no instruments of precision such as we now 
have at our disposal, and so his work gives us 
only a rather rough approximation of the true 
place of the star. A small circle might be im- 
agined on the sky of a size comparable with the 
possible errors of Tycho's observations. We 
could then say with certainty that his star must 
have been situated somewhere within that little 
circle, but it is impossible to know exactly where. 

It happens that our modern telescopes reveal 
the existence of several faint stars within the 
space covered by such a circle. Any one of these 
would have been too small for Tycho to see, and, 
therefore, any one of them may be his once brilliant 
luminary reduced to a state of permanent or tem- 
porary semi-darkness. These considerations are, 
indeed, of great importance in explaining the 



phenomena of temporary stars. If Tycho had 
been able to leave us a more exact determination 
of his star's place in the sky, and even if our most 
powerful instruments could not show anything in 
that place to-day, we might nevertheless theorize 
on the supposition that the object still exists, but 
has reached a condition almost entirely dark. 

Indeed, the latest theory classes temporary stars 
among those known as variable. For many stars 
are known to undergo quite decided changes in 
brilliancy ; possibly inconstancy of light is the rule 
rather than the exception. But while such changes, 
when they exist, are too small to be perceptible 
in most cases, there is certainly a large number of 
observable variables, subject to easily measurable 
alterations of light. Astronomers prefer to see in 
the phenomena of temporary stars simple cases of 
variation in which the increase of light is sudden, 
and followed by a gradual diminution. Possibly 
there is then a long period of comparative or even 
complete darkness, to be followed as before by a 
sudden blazing up and extinction. No temporary 
star, however, has been observed to reappear in the 
same celestial place where once had glowed its 



sudden outburst. But cases are not wanting 
where incandescence has been both preceded and 
followed by a continued existence, visible though 
not brilliant. 

For such cases as these it is necessary to come 
down to modern records. We cannot be sure 
that some faint star has been temporarily brilliant, 
unless we actually see the conflagration itself, or 
are able to make the identity of the object's pre- 
cise location in the sky before and after the event 
perfectly certain by the aid of modern instru- 
ments of precision. But no one has ever seen 
the smouldering fires break out. Temporary 
stars have always been first noticed only after 
having been active for hours if not for days. So 
we must perforce fall back on instrumental iden- 
tification by determinations of the star's exact 
position upon the celestial vault. 

Some time between May loth and lath in the 
year 1866 the ninth star in the list of known 
" temporaries " appeared. It possessed very 
great light-giving power, being surpassed in brill- 
iancy by only about a score of stars in all the 
heavens. It retained a maximum luminosity only 



three or four days, and in less than two months 
had diminished to a point somewhere between the 
ninth and tenth " magnitudes." In other words, 
from a conspicuous star, visible to the naked eye, 
it had passed beyond the power of anything less 
than a good telescope. Fortunately, we had ex- 
cellent star-catalogues before 1866. These were 
at once searched, and it was possible to settle 
quite definitely that a star of about the ninth or 
tenth magnitude had really existed before 1866 
at precisely the same point occupied by the new 
one. Needless to say, observations were made of 
the new star itself, and afterward compared with 
later observations of the faint one that still occu- 
pies its place. These render quite certain the 
identity of the temporary bright star with the 
faint ones that preceded and followed it. 

Such results, on the one hand, offer an excel- 
lent vindication of the painstaking labor expended 
on the construction of star-catalogues, and, on the 
other, serve to elucidate the mystery of tempo- 
rary stars. Nothing can be more plausible than 
to explain by analogy those cases in which no 
previous or subsequent existence has been ob- 



served. It is merely necessary to suppose that, 
instead of varying from the ninth or tenth mag- 
nitude, other temporary objects have begun and 
ended with the twentieth ; for the twentieth mag- 
nitude would be beyond the power of our best 

Nor is the star of 1866 an isolated instance. 
Ten years later, in 1876, a temporary star blazed 
up to about the second magnitude, and returned 
to invisibility, so far as the naked eye is concerned, 
within a month, having retained its greatest brill- 
iancy only one or two days. This star is still 
visible as a tiny point of light, estimated to be of 
the fifteenth magnitude. Whether it existed 
prior to its sudden outburst can never be known, 
because we do not possess catalogues including 
the generality of stars as faint as this one must 
have been. But at all events, the continued ex- 
istence of the object helps to place the temporary 
stars in the class of variables. 

The next star, already mentioned under " neb- 
ula," was first seen in 1885. It was in one re- 
spect the most remarkable of all, for it appeared 
almost in the centre of the great nebula in the 



constellation Andromeda. It was never very 
bright, reaching only the sixth magnitude or 
thereabouts, was observed during a period of only 
six months, and at the end of that time had faded 
beyond the reach of our most powerful glasses. 
It is a most impressive fact that this event oc- 
curred within the nebula. Whatever may be the 
nature of the explosive catastrophe to which the 
temporary stars owe their origin, we can now say 
with certainty that not even those vast elemental 
luminous clouds men call nebulae are free from 

The last outburst on our records was first no- 
ticed February 22, 1901. The star appeared in 
the constellation Perseus, and soon reached the 
first magnitude, surpassing almost every other 
star in the sky. It has been especially remark- 
able in that it has become surrounded by a nebu- 
lous mass in which are several bright condensa- 
tions or nuclei ; and these seem to be in very 
rapid motion. The star is still under observa- 
tion (January, 1902). 



Among the figures that stand out sharply 
upon the dim background of old-time science, 
there is none that excites a keener interest than 
Galileo. Most people know him only as a dis- 
tinguished man of learning ; one who carried on 
a vigorous controversy with the Church on mat- 
ters scientific. It requires some little study, 
some careful reading between the lines of astro- 
nomical history, to gain acquaintance with the 
man himself. He had a brilliant, incisive wit ; 
was a genuine humorist ; knew well and loved 
the amusing side of things ; and could not often 
forego a sarcastic pleasantry, or deny himself the 
pleasure of argument. Yet it is more than 
doubtful if he ever intended impertinence, or 
gave willingly any cause of quarrel to the 

His acute understanding must have seen that 
there exists no real conflict between science and 



religion ; for time, in passing, has made common 
knowledge of this truth, as it has of many things 
once hidden. When we consider events that oc- 
curred three centuries ago, it is easy to replace 
excited argument with cool judgment ; to re- 
member that those were days of violence and 
cruelty ; that public ignorance was of a density 
difficult to imagine to-day ; and that it was uni- 
versally considered the duty of the Church to 
assume an authoritative attitude upon many 
questions with which she is not now required to 
concern herself in the least. Charlatans, unbal- 
anced theorists, purveyors of scientific marvels, 
were all liable to be passed upon definitely by 
the Church, not in a spirit of impertinent inter- 
ference, but simply as part of her regular duties. 
If the Church's judgment in such matters was 
sometimes erroneous ; if her interference now 
and again was cruel, the cause must be sought in 
the manners and customs of the time, when per- 
secution rioted in company with ignorance, and 
violence was the law. Perhaps even to-day it 
would not be amiss to have a modern scientific 
board pass authoritatively upon novel discov- 



eries and inventions, so as to protect the public 
against impostors as the Church tried to do of 

Galileo was born at Pisa in 1564, and his long 
life lasted until 1642, the very year of Newton's 
birth. His most important scientific discoveries 
may be summed up in a few words ; he was the 
first to use a telescope for examining the heav- 
enly bodies ; he discovered mountains on the 
moon ; the satellites of Jupiter ; the peculiar 
appearance of Saturn which Huygens afterward 
explained as a ring surrounding the ball of the 
planet ; and, finally, he found black spots on the 
sun's disk. These discoveries, together with his 
remarkable researches in mechanical science, con- 
stitute Galileo's claim to immortality as an in- 
vestigator. But, as we have said, it is not our 
intention to consider his work as a series of sci- 
entific discoveries. We shall take a more inter- 
esting point of view, and deal with him rather as 
a human being who had contracted the habit of 
making scientific researches. 

What must have been his feelings when he 
first found with his " new " telescope the satel- 



lites of Jupiter ? They were seen on the night 
of January 7, 16 10. He had already viewed the 
planet through his earlier and less powerful 
glass, and was aware that it possessed a round 
disk like the moon, only smaller. Now he saw 
also three objects that he took to be little stars 
near the planet. But on the following night, as 
he says, " drawn by what fate I know not," the 
tube was again turned upon the planet. The 
three small stars had changed their positions, and 
were now all situated to the west of Jupiter, 
whereas on the previous night two had been on 
the eastern side. He could not explain this 
phenomenon, but he recognized that there was 
something peculiar at work. Long afterward, in 
one of his later works, translated into quaint old 
English by Salusbury, he declared that " one sole 
experiment sufficeth to batter to the ground a 
thousand probable Arguments." This was al- 
ready the guiding principle of his scientific activ- 
ity, a principle of incomparable importance, and 
generally credited to Bacon. Needless to say, 
Jupiter was now examined every night. 

The 9th was cloudy, but on the loth he again 


saw his little stars, their number now reduced to 
two. He guessed that the third was behind the 
planet's disk. The position of the two visible 
ones was altogether different from either of the 
previous observations. On the nth he became 
sure that what he saw was really a series of sat- 
ellites accompanying Jupiter on his journey 
through space, and at the same time revolving 
around him. On the 12th, at 3 a.m., he actu- 
ally saw one of the small objects emerge from 
behind the planet; and on the 13th he finally 
saw four satellites. Two hundred and eighty- 
two years were destined to pass away before any 
human eye should see a fifth. It was Barnard in 
1892 who followed Galileo. 

To understand the eflFect of this discovery 
upon Galileo requires a person who has himself 
watched the stars, not, as a dilettante, seeking 
recreation or amusement, but with that deep rev- 
erence that comes only to him who feels — nay, 
knows — that in the moment of observation just 
passed he too has added his mite to the great 
fund of human knowledge. Galileo's mummied 
forefinger still points toward the stars from its 



little pedestal of wood in the Museo at Florence, 
a sign to all men that he is unforgotten. But 
Galileo knew on that nth of January, 1610, that 
the memory of him would never fade ; that the 
very music of the spheres would thenceforward 
be attuned to a truer note, if any would but 
hearken to the Jovian harmony. For he recog- 
nized at once that the visible revolution of these 
moons around Jupiter, while that planet was 
himself visibly travelling through space, must 
deal its death-blow to the old Ptolemaic system 
of the universe. Here was a great planet, the 
centre of a system of satellites, and yet not 
the centre of the universe. Surely, then, the 
earth, too, might be a mere planet like Jupiter, 
and not the supposed motionless centre of all 

The satellite discovery was published in 16 10 
in a little book called " Sidereus Nuncius," usually 
translated " The Sidereal Messenger." It seems 
to us, however, that the word " messenger " is 
not strong enough ; surely in Papal Italy a nun- 
cius was more than a mere messenger. He was 
clothed with the very highest authority, and we 



think it probable that Galileo's choice of this 
word in the title of his book means that he 
claimed for himself similar authority in science. 
At all events, the book made him at once a great 
reputation and numerous enemies. 

But it was not until 1616 that the Holy Office 
(Inquisition) issued an edict ordering Galileo to 
abandon his opinion that the earth moved, and at 
the same time placed Copernicus's De Revolutioni- 
bus and two other books advocating that doc- 
trine on the " Index Librorum Prohibitorum," 
or list of books forbidden by the Church. These 
volumes remained in subsequent editions of the 
"Index" down to 1821, but they no longer 
appear in the edition in force to-day. 

Galileo's most characteristic work is entitled 
the " Dialogue on the Two Chief Systems of the 
World." It was not published until 1632, 
although the idea of the book was conceived 
many years earlier. In it he gave full play to his 
extraordinary powers as a true humorist, a fine 
lame among controversialists, and a genuine 
man of science, valuing naked truth above all 
other things. As may be imagined, it was no 



small matter to obtain the authorities' consent to 
this publication. Galileo was already known to 
hold heretical opinions, and it was suspected that 
he had not laid them aside when commanded to 
do so by the edict of 1616. But perhaps Gali- 
leo's introduction to the " Dialogue " secured the 
censor's imprimatur; it is even suspected that 
the Roman authorities helped in the preparation 
of this introduction. Fortunately, we have a de- 
lightful contemporary translation into English, 
by Thomas Salusbury, printed at London by 
Leybourne in 1661. We have already quoted 
from this translation, and now add from the same 
work part of Galileo's masterly preface to the 
" Dialogue " : 

" Judicious reader, there was published some 
years since in Rome a salutiferous Edict, that, for 
the obviating of the dangerous Scandals of the 
Present Age, imposed a reasonable Silence upon 
the Pythagorean (Copernican) opinion of the 
Mobility of the Earth. There want not such 
as unadvisedly affirm, that the Decree was not 
the production of a sober Scrutiny, but of an 
ill-formed passion ; and one may hear some 



mutter that Consultors altogether ignorant of 
Astronomical observations ought not to clipp 
the wings of speculative wits with rash prohi- 

Galileo first states his own views, and then 
pretends that he will oppose them. He goes on 
to say that he believes in the earth's immobility, 
and takes " the contrary only for a mathematical 
Capriccio" as he calls it ; something to be con- 
sidered, because possessing an academical interest, 
but on no account having a real existence. Of 
course any one (even a censor) ought to be able 
to see that it is the Capriccio, and not its oppo- 
site, that Galileo really advocates. Three persons 
appear in the " Dialogue " : Salviati, who be- 
lieves in the Copernican system; Simplicio, of 
suggestive name, who thinks the earth cannot 
move ; and, finally, Sagredus, a neutral gentle- 
man of humorous propensities, who usually be- 
gins by opposing Salviati, but ends by being con- 
vinced. He then helps to punish poor Simplicio, 
who is one of those persons apparently incapable 
of comprehending a reasonable argument. Here 
is an interesting specimen of the " Dialogue " 



taken from Salusbury's translation : Salviati refers 
to the argument, then well known, that the earth 
cannot rotate on its axis, " because of the impos- 
sibility of its moving long without wearinesse." 
Sagredus replies : " There are some kinds of 
animals which refresh themselves after wearinesse 
by rowling on the earth ; and that therefore there 
is no need to fear that the Terrestrial Globe 
should tire, nay, it may be reasonably affirmed 
that it enjoyeth a perpetual and most tranquil re- 
pose, keeping itself in an eternal rowling." Sal- 
viati's comment on this sally is, " You are too 
tart and satyrical, Sagredus." 

There is no doubt that the " Dialogue " fin- 
ished the Ptolemaic theory, and made that of 
Copernicus the only possible one. At all events, 
it brought about the well-known attack upon 
Galileo from the authorities of the Holy Office. 
We shall not recount the often-told tale of his 
recantation. He was convicted (very rightly) of 
being a Copernican, and was forced to abjure that 
doctrine. Galileo's life may be summed up as 
one of those through which the world has been 
made richer. A clean-cutting analytic wit, never 



becoming dull : heated again and again in the 
fierce blaze of controversy, it was allowed to cool 
only that it might acquire a finer temper, to 
pierce with fatal certainty the smallest imperfec- 
tions in the armor of his adversaries. 



The discovery of a new and important planet 
usually receives more immediate popular atten- 
tion and applause than any other astronomical 
event. Philosophers are fond of referring to our 
solar system as a mere atom among the countless 
universes that seem to be suspended within the 
profound depths of space. They are wont to 
point out that this solar system, small and in- 
significant as a whole in comparison with many 
of the stellar worlds, is, nevertheless, made up of 
a large number of constituent planets ; and these 
in turn are often accompanied with still smaller 
satellites, or moons. Thus does Nature provide 
worlds within worlds, and it is not surprising 
that public attention should be at once attracted 
by any new member of our sun's own special 
family of planets. The ancients were acquainted 
with only five of the bodies now counted as 
planets, viz. : Mercury, Venus, Mars, Jupiter, 



and Saturn. The dates of their discovery are 
lost in antiquity. To these Uranus was added 
in 1 78 1 by a brilliant effort of the elder Herschel. 
We are told that intense popular excitement 
followed the announcement of Herschel's first 
observation : he was knighted and otherwise 
honored by the English King, and was enabled 
to lay a secure foundation for the future distin- 
guished astronomical reputation of his family. 

Herschel's discovery quickened the restless 
activity of astronomers. Persistent efforts were 
made to sift the heavens more and more closely, 
with the strengthened hope of adding still further 
to our planetary knowledge. An association 
of twenty-four enthusiastic German astronomers 
was formed for the express purpose of hunt- 
ing planets. But it fell to the lot of an Ital- 
ian, Piazzi, of Palermo, to find the first of that 
series of small bodies now known as the asteroids 
or minor planets. He made the discovery at the 
very beginning of our century, January i, 1801. 

But news travelled slowly in those days, and it 
was not until nearly April that the German observ- 
ers heard from Piazzi. In the meantime, he had 



himself been prevented by illness from continu- 
ing his observations. Unfortunately, the planet 
had by this time moved so near the sun, on ac- 
count of its own motions and those of the earth, 
that it could no longer be observed. The bright 
light of the sun made observations of the new 
body impossible ; and it was feared that, owing 
to lack of knowledge of the planet's orbit, astron- 
omers would be unable to trace it. So there 
seemed, indeed, to be danger of an almost irrep- 
arable loss to science. But in scientific, as in 
other human emergencies, someone always ap- 
pears at the proper moment. A very young 
mathematician at Gottingen, named Gauss, at- 
tacked the problem, and was able to devise a 
method of predicting the future course of the 
planet on the sky, using only the few observa- 
tions made by Piazzi himself. Up to that time 
no one had attempted to compute a planetary 
orbit, unless he had at his disposal a series of 
observations extending throughout the whole 
period of the planet's revolution around the sun. 
But the Piazzi planet offered a new problem in 
astronomy. It had become imperatively neces- 



sary to obtain an orbit from a few observations 
made at nearly the same date. Gauss's work was 
signally triumphant, for the planet was actually 
found in the position predicted by him, as soon 
as a change in the relative places of the planet 
and earth permitted suitable observations to be 

But after all, Piazzi's planet belongs to a class 
of quite small bodies, and is by no means as in- 
teresting as Herschel's discovery, Uranus. Yet 
even this must be relegated to second rank 
among planetary discoveries. On September 23, 
1846, the telescope of the Berlin Observatory 
was directed to a certain point on the sky for a 
very special reason. Galle, the astronomer of 
Berlin, had received a letter from Leverrier, of 
Paris, telling him that if he would look in a certain 
direction he would detect a new and large planet. 

Leverrier's information was based upon a math- 
ematical calculation. Seated in his study, with,, ,Yr'_^^^9 9 y 
no instruments but pen and paper, he had slowly 
figured out the history of a world as yet un- 
seen. Tiny discrepancies existed in the observed 
motions of Herschel's planet Uranus. No man 



had explained their cause. To Leverrier's acute 
understanding they slowly shaped themselves 
into the possible effects of attraction emanating 
from some unknown planet exterior to Uranus. 
Was it conceivable that these slight tremulous 
imperfections in the motion of a planet could be 
explained in this way ? Leverrier was able to 
say confidently, "Yes." But we may rest as- 
sured that Galle had but small hopes that upon 
his eye first, of all the myriad eyes of men, would 
fall a ray of the new planet's light. Careful 
and methodical, he would neglect no chance of 
advancing his beloved science. He would look. 
Only one who has himself often seen the morn- 
ing's sunrise put an end to a night's observation 
of the stars can hope to appreciate what Galle's 
feelings must have been when he saw the planet. 
To his trained eye it was certainly recognizable 
at once. And then the good news was sent on 
to Paris. We can imagine Leverrier, the cool 
calculator, saying to himself: " Of course he 
found it. It was a mathematical certainty." 
Nevertheless, his satisfaction must have been 
of the keenest. No triumphs give a pleasure 



higher than those of the intellect. Let no one 
imagine that men who make researches in the 
domain of pure science are under-paid. They 
find their reward in pleasure that is beyond any 

The Leverrier planet was found to be the last 
of the so-called major planets, so far as we can 
say in the present state of science. It received 
the name Neptune. Observers have found no 
other member of the solar system comparable in 
size with such bodies as Uranus and Neptune. 
More than one eager mathematician has tried to 
repeat Leverrier's achievement, but the supposed 
planet was not found. It has been said that fig- 
ures never lie ; yet such is the case only when 
the computations are correctly made. People 
are prone to give to the work of careless or in- 
competent mathematicians the same degree of 
credence that is really due only to masters of the 
craft. It requires the test of time to affix to any 
man's work the stamp of true genius. 

While, then, we have found no more large 
planets, quite a group of companions to Piazzi's 
little one have been discovered. They are all 



small, probably never exceeding about 400 miles 
in diameter. All travel around the sun in orbits 
that lie wholly within that of Jupiter and are ex- 
terior to that of Mars. The introduction of as- 
tronomical photography has given a tremendous 
impetus to the discovery of these minor planets, 
as they are called. It is quite interesting to ex- 
amine the photographic process by which such 
discoveries are made possible and even easy. 
The matter will not be difficult to understand if 
we remember that all the planets are continually 
changing their places among the other stars. For 
the planets travel around the sun at a compara- 
tively small distance. The great majority of the 
stars, on the contrary, are separated from the sun 
by an almost immeasurable space. As a result, 
they do not seem to move at all among them- 
selves, and so we call them fixed stars : they may, 
indeed, be in motion, but their great distance pre- 
vents our detecting it in a short period of time. 

Now, stellar photographs are made in much 
the same way as ordinary portraits. Only, in- 
stead of using a simple camera, the astronomer 
exposes his photographic plate at the eye-end of 



a telescope. The sensitive surface of the plate is 
substituted for the human eye. We then find 
on the picture a little dot corresponding to every 
star within the photographed region of the sky. 
But, as everyone knows, the turning of the 
earth on its axis makes the whole heavens, in- 
cluding the sun, moon, and stars, rise and set 
every day. So the stars, when we photograph 
them, are sure to be either climbing up in the 
eastern sky or else slowly creeping down in the 
western. And that makes astronomical photog- 
raphy very different from ordinary portrait work. 
The stars correspond to the sitter, but they 
don't sit still. For this reason it is necessary to 
connect the telescope with a mechanical contriv- 
ance which makes it turn round like the hour- 
hand of an ordinary clock. The arrangement is 
so adjusted that the telescope, once aimed at the 
proper object in the sky, will move so as to re- 
main pointed exactly the same during the whole 
time of the photographic exposure. Thus, while 
the light of any star is acting on the plate, such 
action will be continuous at a single point. 
Consequently, the finished picture will show the 



star as a little dot; while without this arrange* 
ment, the star would trail out into a line in- 
stead of a dot. Now we have seen that the 
planets are all moving slowly among the fixed 
stars. So if we make a star photograph in a part 
of the sky where a planet happens to be, the 
planet will make a short line on the plate ; 
whereas, if the planet remained quite unmoved 
relatively to the stars it would give a dot like 
the star dots. The presence of a line, therefore, 
at once indicates a planet. 

This method of planet-hunting has proved 
most useful. More than 400 small planets sim- 
ilar to Piazzi's have been found, though never 
another one like Uranus and Neptune. As 
we have said, all these little bodies lie between 
Mars and Jupiter. They evidently belong to a 
group or family, and many astronomers have 
been led to believe that they are but fragments 
of a former large planet. 

In August, 1898, however, one was found by 
Witt, of Berlin, which will probably occupy a 
very prominent place in the annals of astronomy. 
For this planet goes well within the orbit of 



Mars, and this will bring it at times very close 
to the earth. In fact, when the motions of the 
new planet and the earth combine to bring them 
to their positions of greatest proximity, the new 
planet will approach us closer than any other 
celestial body except our own moon. Witt 
named his new planet Eros. Its size, though 
small, may prove to be sufficient to bring it within 
the possibilities of naked-eye observation at the 
time of closest approach to the earth. 

To astronomers the great importance of this 
new planet is due to the following circumstance : 
For certain reasons too technical to be stated 
here in detail, the distance from the earth to any 
planet can be determined with a degree of pre- 
cision which is greatest for planets that are near 
us. Thus in time we shall learn the distance of 
Eros more accurately than we know any other 
celestial distance. From this, by a process of 
calculation, the solar distance from the earth is 
determinable. But the distance from earth to 
sun is the fundamental astronomical unit of meas- 
ure ; so that Witt's discovery, through its effect 
on the unit of measure, will doubtless influence 



every part of the science of astronomy. Here 
we have once more a striking instance of the 
reward sure to overtake the diligent worker in 
science — a whole generation of men will doubt- 
less pass away before we shall have exhausted 
the scientific advantages to be drawn from Witt's 
remarkable observation of 1898. 



Long before clocks and watches had been in- 
vented, people began to measure time with sun- 
dials. Nowadays, when almost everyone has a 
watch in his pocket, and can have a clock, too, 
on the mantel-piece of every room in the house, 
the sun-dial has ceased to be needed in ordinary 
life. But it is still just as interesting as ever to 
anyone who would like to have the means of 
getting time direct from the sun, the great hour- 
hand or timekeeper of the sky. Any person 
who is handy with tools can make a sun-dial 
quite easily, by following the directions given 

In the first place, you must know that the 
sun-dial gives the time by means of the sun's 
shadow. If you stick a walking-cane up in the 
sand on a bright, sunshiny day, the cane has a 

* This chapter is especially intended for boys and girls and others who like 
to make things with carpenters* tools. 



long shadow that looks like a dark line on the 
ground. Now if you watch this shadow care- 
fully, you will see that it does not stay in the 
same place all day. Slowly but surely, as the 
sun climbs up in the sky, the shadow creeps 
around the cane. You can see quite easily that 
if the cane were fastened in a board floor, and if 
we could mark on the floor the places where the 
shadow was at different hours of the day, we 
could make the shadow tell us the time just like 
the hour-hand of a clock. A sun-dial is just 
such an arrangement as this, and I will show you 
how to mark the shadow places exactly, so as to 
tell the right time without any trouble whenever 
the sun shines. 

If you were to watch very carefully such an 
arrangement as a cane standing in a board floor, 
you would not find the creeping shadow in just 
the same place at the same time every day. If 
you marked the place of the shadow at exactly 
ten o'clock by your watch some morning, and 
then went back another day at ten, you would 
not find the shadow on the old mark. It would 
not get very far from it in a day or two, but in a 



month or so it would be quite a distance away. 
Now, of course, a sun-dial would be of no use if 
it did not tell the time correctly every day ; and 
in fact, it is not easy to make a dial when the 
shadow is cast by a stick standing straight up. 
But we can get over this difficulty very well by 
letting the shadow be cast by a stick that leans 
over toward the floor just the right amount, as I 
will explain in a moment. Of course, we should 
not really use the floor for our sun-dial. It is 
much better to mark out the hour-lines, as they 
are called, on a smooth piece of ordinary white 
board, and then, after the dial is finished, it can 
be screwed down to a piazza floor or railing, or it 
can be fastened on a window-sill. It ought to 
be put in a place where the sun can get at it 
most of the time, because, of course, you cannot 
use the sun-dial when the sun is not shining on 
it. If the dial is set on a window-sill (of a city 
house, for instance) you must choose a south 
window if you can, so as to get the sun nearly all 
day. If you have to take an east window, you 
can use the dial in the morning only, and in a 
west window only in the afternoon. Sometimes 



it is best not to try to fasten the dial to its 
support with screws, but just to mark its place, 
and then set it out whenever you want to use 
it. For if the dial is made of wood, and not 

Fig. I. 

painted, it might be injured by rain or snow 

in bad weather if left out on a window-sill or 


It is not quite easy to fasten a little stick to a 

board so that it will lean over just right. So it 

is better not to use a stick or a cane in the way 

I have described, but instead to use a piece of 

board cut to just the right shape. 

Fig. I shows what a sun-dial should look like. 


The lines to show the shadow's place at the dif- 
ferent hours of the day will be marked on the 
board ABCD, and this will 
be put flat on the window-sill y^ 

or piazza floor. The three- / 

cornered piece of board abc y 
is fastened to the bottom- « '""" "^ 

board ABCD by screws going 
through ABCD from underneath. The edge ab 
of the three-cornered board abc then takes the 
place of the leaning stick or cane, and the time is 
marked by the shadow cast by the edge ab. Of 
course, it is important that this edge should be 
straight and perfectly flat and even. If you are 
handy with tools, you can make it quite easily, 
but if not, you can mark the right shape on a 
piece of paper very carefully, and take it to a 
carpenter, who can cut the board according to 
the pattern you have marked on the paper. 

Now I must tell you how to draw the shape 
of the three-cornered board abc. Fig. i shows 
how it is done. The side ac should always be 
just five inches long. The side be is drawn at 
right angles to ac^ which you can do with an or- 



dinary carpenter's square. The length of be de- 
pends on the place for which the dial is made. 
The following table gives the length of be for 
various places in the United States, and, after you 
have marked out the length of be^ it is only 
necessary to complete the three-cornered piece by 
drawing the side ab from a to b. 

Table Showing the Length of the Side he. 





4 11-16 


4 1-16 


4 1-2 


4 11-16 


3 '-4 

4 1-1 


4 1-16 


4 i-» 


4. ^-16 



4 1-16 

Kansas City 

3 15-16 


3 15-16 


3 11-16 

New Orleans 

2 7-8 



New York 4 3-g 

Omaha 4 3-8 

Philadelphia 4 3-16 

Pittsburg 4 3-8 

Portland, Me 4 13-16 

Richmond _.3 15-16 

Rochester 4 11-16 

San Diego 3 1.4 

San Francisco 3 15-16 

Savannah 3 1-8 

St. Louis 3 15-16 

St. Paul 5 

Seattle 5 9-16 

Washington, D. C 4 i-i6 

Tthaca ^ 9_/^ 

If you wish to make a dial for a place not given 
in the table, it will be near enough to use the dis- 
tance be as given for the place nearest to you 
But in selecting the nearest place from the table, 
please remember to take that one of the cities 
mentioned which is nearest to you in a north-and- 



south direction. It does not matter how far away 
the place is in an east-and-west direction. So, in- 
stead of taking the place that is nearest to you on 
the map in a straight line, take the place to which 
you could travel by going principally east or west, 
and very little north or south. The figure drawn 
is about the right shape for New York. The 
board used for the three-cornered piece should be 
about one-half inch thick. But if you are making 
a window-sill dial, you may prefer to have it 
smaller than I have described. You can easily 
have it half as big by making all the sizes and 
lines in half-inches where the table calls for 

After you have marked out the dimensions for 
the three-cornered piece that is to throw the 
shadow, you can prepare the dial itself, with the 
lines that mark the place of the shadow for every 
hour of the day. This you can do in the manner 
shown in Fig. 3. Just as in the case of the three- 
cornered piece, you can draw the dial with a pencil 
directly on a smooth piece of white board, about 
three-quarters of an inch thick, or you can mark 
it out on a paper pattern and transfer it afterward 



to the board. Perhaps it will be as well to begin 
by drawing on paper, as any mistakes can then 
be corrected before you commence to mark your 

In the first place you must draw a couple of 


Fig. J. 

lines MN and M'N', eight inches long, and just 
far enough apart to fit the edge of your three- 
cornered shadow-piece. You will remember I 
told you to make that one-half inch thick, so 
your two lines will also be one-half inch apart. 
Now draw the two lines NO and N'O' square 



with MN and M'N', and make the distances 
NO and N'O' just five inches each. The lines 
OK, O'K', and the other lines forming the outer 
border of the dial, are then drawn just as shown, 
OK and O'K' being just eight inches long, the 
same as MN and M'N'. The lower lines in 
the figure, which are not very important, are to 
complete the squares. You must mark the lines 
NO and N'O' with the figures VI, these being 
the lines reached by the shadow at six o'clock in 
the morning and evening. The points where the 
VII, VIII, and other hour-lines cut the lines 
OK, O'K', MK, and M'K' can be found from 
the table on page 78. 

In using the table you will notice that the line 
IX falls sometimes on one side of the corner K, 
and sometimes on the other. Thus for Albany 
the line passes seven and seven-sixteenth inches 
from O, while for Charleston it passes four and 
three-eighth inches from M. For Baltimore it 
passes exactly through the corner K. 

The distance for the line marked V from O' is 
just the same as the distance from O to VII. 
Similarly, IV corresponds to VIII, III to IX, 



Table Showing How to Mark the Hour-lines. 













Kansas City 



New Orleans 

New York 




Portland, Me 



San Diego 

San Francisco 


St. Louis 

St. Paul 


Washington, D. C 

Distance from O to the line 


I 15-16 










4 3-16 

4 11-16 

4 S-x6 




















7 7-'6 


7 7-16 


7 7-16 

7 11-16 

7 7-16 

7 7-16 

7 11-16 

7 11-16 

7 11-16 

7 11-16 

7 1-8 


7 7-16 

7 1-8 
6 5-8 

7 n-'i 

Distance from M to the line 



4 3-8 

4 1-16 

4 3-« 
4 1-4 



3 1-16 

2 7-8 

3 1-16 
3 1-16 

2 1-2 

3 1-16 

2 7-8 

3 1-16 

2- 7-8 

3 1-16 

2 7-8 

2 7-8 

2 7-8 

3 1-16 

2 5-16 

3 1-16 
3 1-16 

2 7-8 

3 1-16 
3 3-16 

2 7-8 

3 1-16 
2 1-2 
2 7-8 
2 1-2 

2 7-8 

3 3-j6 
3 3-8 
2 7-8 
^ /-? 
































II to X, and I to XI. The number XII is 
marked at MM ' as shown. If you desire to add 
lines (not shown in Fig. 3 to avoid confusion) for 
hours earlier than six in the morning, it is merely 
necessary to mark off a distance on the line KO, 
below the point O, and equal to the distance from 
O to VII. This will give the point where the 



5 A.M. shadow line drawn from N cuts the line 
KO. A corresponding line for 7 p.m. can be 
drawn from N' on the other side of the figure. 

After you have marked out the dial very care- 
fully, you must fasten the three-cornered shadow- 
piece to it in such a way that the whole instru- 
ment will look like Fig. i. The edge ac (Fig. 
a) goes on NM (Fig. 3). The point a (Fig. 2) 
must come exactly on N (Fig. 3) ; and as the 
lines NM (Fig. 3) and N'M' (Fig. 3) have been 
made just the right distance apart to fit the 
thickness of the three-cornered piece ahc (Fig. 
a), everything will go together just right. The 
point c (Fig. 2) will not quite reach to M (Fig. 3), 
but will be on the line NM (Fig. 3) at a distance 
of three inches from M. The two pieces of 
wood will be fastened together with three screws 
going through the bottom-board ABCD (Figs. 
I and 3) and into the edge ac (Fig. 2) of the 
three-cornered piece. The whole instrument will 
then look something like Fig. i. 

After you have got your sun-dial put together, 
you need only set it in the sun in a level place, 
on a piazza or window-sill, and turn it round 



until it tells the right time by the shadow. You 
can get your local time from a watch near enough 
for setting up the dial. Once the dial is set right 
you can screw it down or mark its position, and 
it will continue to give correct solar time every 
day in the year. 

If you wish to adjust the dial very closely, 
you must go out some fine day and note the er- 
ror of the dial by a watch at about ten in the 
morning, and at noon, and again at about two in 
the afternoon. If the error is the same each time, 
the dial is rightly set. If not, you must try, by 
turning the dial slightly, to get it so placed that 
your three errors will be nearly the same. When 
you have got them as nearly alike as you can, the 
dial will be sufficiently near right. The solar or 
dial time may, however, diflFer somewhat from 
ordinary watch time, but the difi^erence will never 
be great enough to matter, when we remember 
that sun-dials are only rough timekeepers after 
all, and usefiil principally for amusement. 



New highways of science have been monu- 
mented now and again by the masterful efforts of 
genius, working single-handed ; but more often 
it is slow-moving time that ripens discovery, and, 
at the proper moment, opens some new path to 
men whose intellectual power is but willingness 
to learn. So the annals of astronomical photog- 
raphy do not recount the achievements of extra- 
ordinary genius. It would have been strange, 
indeed, if the discovery of photography had not 
been followed by its application to astronomy. 

The whole range of chemical science contains 
no experiment of greater inherent interest than 
the development of a photographic plate. Let 
but the smallest ray of light fall upon its 
strangely sensitive surface, and some subtle invis- 
ible change takes place. It is then merely nec- 
essary to plunge the plate into a properly pre- 
pared chemical bath, and the gradual process of 


developing the picture begins. Slowly, very 
slowly, the colorless surface darkens wherever 
light has touched it. Let us imagine that the 
exposure has been made with an ordinary lens 
and camera, and that it is a landscape seeming to 
grow beneath the experimenter's eyes. At first 
only the most conspicuous objects make their 
appearance. But gradually the process extends, 
until finally every tiny detail is reproduced with 
marvellous fidelity to the original. The photo- 
graphic plate, when developed in this way, is 
called a " negative." For in Nature luminous 
points, or sources of light, are bright, while the 
developing negative turns dark wherever light 
has acted. Thus the negative, while true to Nat- 
ure, reproduces everything in a reversed way; 
bright things are dark, and shadows appear light. 
For ordinary purposes, therefore, the negative has 
to be replaced by a new photograph made by copy- 
ing it again photographically. In this way it is 
again reversed, giving us a picture corresponding 
correctly to the facts as seen. Such a copy from 
a negative is what is ordinarily called a photo- 
graph ; technically, it is known as a " positive." 



One of the remarkable things about the sensi- 
tive plate is its complete indifference to the dis- 
tance from which the light comes. It is ready 
to yield obediently to the ray of some distant 
star that may have journeyed, as it were, from 
the very vanishing, point of space, or to the 
bright glow of an electric light upon the photog- 
rapher's table. This quality makes its use es- 
pecially advantageous in astronomy, since we can 
gain knowledge of remote stars only by a study 
of the light they send us. In such study the 
photographic plate possesses a supreme advan- 
tage over the human eye. If the conditions of 
weather and atmosphere are favorable, an ob- 
server looking through an ordinary telescope 
will see nearly as much at the first glance as he 
will ever see. Attentive and continued study will 
enable him to fix details upon his memory, and 
to record them by means of drawings and dia- 
grams. Occasional moments of especially un- 
disturbed atmospheric conditions will allow him 
to glimpse faint objects seldom visible. But on 
the whole, telescopic astronomers add little to 
their harvest by continued husbandry in the 



same field of stars. Photography is different. 
The effect of light upon the sensitive surface of 
the plate is strictly cumulative. If a given star 
can bring about a certain result when it has been 
allowed to act upon the plate for one minute, 
then in two or three minutes it will accomplish 
much more. Perhaps a single minute's exposure 
would have produced a mark scarcely perceptible 
upon the developed negative. In that case, 
three or four minutes would give us a perfectly 
well defined black image of the star. 

Thus, by lengthening the exposure we can 
make the fainter stars impress themselves upon 
the plate. If their light is not able to produce 
the desired effect in minutes, we can let its action 
accumulate for hours. In this manner it be- 
comes possible and easy to photograph objects 
so faint that they have never been seen, even 
with our most powerful telescopes. This 
achievement ranks high among those which 
make astronomy appeal so strongly to the imag- 
ination. Scientific men are not given to fancies ; 
nor should they be. But the first long-exposure 
photograph must have been an exciting thing. 


Star-Field in Constellation Monoceros. 

Photographed by Barnard, February i, 1894. 

Exposure, three hours. 


After coming from the observatory, the chemical 
development was, of course, made in a dark 
room, so that no additional light might harm the 
plate until the process was complete. Carrying 
it out then into the light, that early experi- 
menter cannot but have felt a thrill of triumph ; 
for his hand held a true picture of dim stars to 
the eye unlighted, lifted into view as if by magic. 
Plates have been thus exposed as long as 
twenty-five hours, and the manner of doing it is 
very interesting. Of course, it is impossible to 
carry on the work continuously for so long a 
period, since the beginning of daylight would 
surely ruin the photograph. In fact, the astron- 
omer must stop before even the faintest streak of 
dawn begins to redden the eastern sky. More- 
over, making astronomical negatives requires ex- 
cessively close attention, and this it is impossible 
to give continuously during more than a few 
hours. But the exposure of a single plate can be 
extended over several nights without difficulty. 
It is merely necessary to close the plate-holder 
with a " light-tight " cover when the first night's 
work is finished. To begin further exposure of 



the same plate on another night, we simply aim 
the photographic telescope at precisely the same 
point of the sky as before. The light-tight 
plate-holder being again opened, the exposure 
can go on as if there had been no interruption. 

Astronomers have invented a most ingenious 
device for making sure that the telescope's aim 
can be brought back again to the same point with 
great exactness. This is a very important mat- 
ter ; for the slightest disturbance of the plate be- 
fore the second or subsequent portions of the ex- 
posure would ruin everything. Instead of a very 
complete single picture, we should have two partial 
ones mixed up together in inextricable confusion. 

To prevent this, photographic telescopes are 
made double, not altogether unlike an opera-glass. 
One of the tubes is arranged for photography 
proper, while the other is fitted with lenses suitable 
for an ordinary visual telescope. The two tubes 
are made parallel. Thus the astronomer, by look- 
ing through the visual glass, can watch objects in 
the heavens even while they are being photo- 
graphed. The visual half of the instrument is 
provided with a pair of very fine cross-wires mov- 



able at will in the field of view. These can be 
made to bisect some little star exactly, before be- 
ginning the first night's work. A fterward, every- 
thing about the instrument having been left un- 
changed, the astronomer can always assure himself 
of coming back to precisely the same point of the 
sky, by so adjusting the instrument that the same 
little star is again bisected. 

It must not be supposed, however, that the 
entire instrument remains unmoved, even during 
the whole of a single night's exposure. For in 
that case, the apparent motion of the stars as they 
rise or set in the sky would speedily carry them 
out of the telescope's field of view. Consequent- 
ly, this motion has to be counteracted by shift- 
ing the telescope so as to follow the stars. This 
can be accomplished accurately and automatically 
by means of clock-work mechanism. Such con- 
trivances have already been applied in the past to 
visual telescopes, because even then they facili- 
tated the observer's work. They save him the 
trouble of turning his instrument every few min- 
utes, and allow him to give his undivided atten- 
tion to the actual business of observation. 



For photographic purposes the telescope needs 
to " follow " the stars far more accurately than in 
the older kind of observing with the eye. Nor is 
it possible to make a clock that will drive the in- 
strument satisfactorily and quite automatically. 
But by means of the second or visual telescope, 
astronomers can always ascertain whether the 
clock is working correctly at any given moment. 
It requires only a glance at the little star bisected 
by the cross-wires, and, if there has been the 
slightest imperfection in the following by clock- 
work, the star will no longer be cut exactly by 
the wires. 

The astronomer can at once correct any error 
by putting in operation a very ingenious me- 
chanical device sometimes called a "mouse- 
control." He need only touch an electric but- 
ton, and a signal is sent into the clock-work. 
Instantly there is a shifting of the mechanism. 
For one of the regular driving wheels is substi- 
tuted, temporarily, another having an extra tooth. 
This makes the clock run a little faster so long 
as the electric current passes. In a similar way, 
by means of another button, the clock can be 



made to run slower temporarily. Thus by 
watching the cross-wires continuously, and ma- 
nipulating his two electric buttons, the photo- 
graphic astronomer can compel his telescope to 
follow exactly the object under observation, and 
he can make certain of obtaining a perfect neg- 

These long-exposure plates are intended espe- 
cially for what may be called descriptive astron- 
omy. With them, as we have seen, advantage is 
taken of cumulative light-effects on the sensitive 
plate, and the telescope's light - gathering and 
space - penetrating powers are vastly increased. 
We are enabled to carry our researches far be- 
yond the confines of the old visible universe. 
Extremely faint objects can be recorded, even 
down to their minutest details, with a fidelity un- 
known to older visual methods. But at present 
we intend to consider principally applications 
of photography in the astronomy of measure- 
ment, rather than the descriptive branch of our 
subject. Instead of describing pictures made 
simply to see what certain objects look like in 
the sky, we shall consider negatives intended for 



precise measurement, with all that the word pre- 
cision implies in celestial science. 

Taking up first the photography of stars, we 
must begin by mentioning the work of Ruther- 
furd at New York. More than thirty years ago 
he had so far perfected methods of stellar pho- 
tography that he was able to secure excellent 
pictures of stars as faint as the ninth magnitude. 
In those days the modern process of dry-plate 
photography had not been invented. To-day, 
plates exposed in the photographic telescope are 
made of glass covered with a perfectly dry film 
of sensitized gelatine. But in the old wet-plate 
process the sensitive film was first wetted with a 
chemical solution ; and this solution could not be 
allowed to dry during the exposure. Conse- 
quently, Rutherfurd was limited to exposures a 
few minutes in length, while nowadays, as we 
have said, their duration can be prolonged at will. 

When we add to this the fact that the old 
plates were far less sensitive to light than those 
now available, it is easy to see what were the diffi- 
culties in the way of photographing faint stars 

in Rutherfurd's time. Nor did he possess the 



modern ingenious device of a combined visual 
and photographic instrument. He had no elec- 
tric controlling apparatus. In fact, the younger 
generation of astronomers can form no adequate 
idea of the patience and personal skill Ruther- 
fiird must have had at his command. For he 
certainly did produce negatives that are but little 
inferior to the best that can be made to-day. 
His only limitation was that he could not 
obtain images of stars much below the ninth 

To understand just what is meant here by the 
ninth magnitude, it is necessary to go back in im- 
agination to the time of Hipparchus, the father 
of sidereal astronomy. (See page 39.); He 
adopted the convenient plan of dividing all the 
stars visible to the naked eye (of course, he had 
no telescope) into six classes, according to their 
brilliancy. The faintest visible stars were put in 
the sixth class, and all the others were assigned 
somewhat arbitrarily to one or the other of the 
brighter classes. 

Modern astronomers have devised a more sci- 
entific system, which has been made to conform 



very nearly to that of Hipparchus, just as it has 
come down to us through the ages. We have 
adopted a certain arbitrary degree of luminosity 
as the standard " first-magnitude"; compared with 
sunlight, this may be represented roughly by a 
fraction of which the numerator is i , and the de- 
nominator about eighty thousand millions. The 
standard second- magnitude star is one whose 
light, compared with a first-magnitude, may be 
represented approximately by the fraction f. 
The third magnitude, in turn, may be compared 
with the second by the same fraction f ; and so 
the classification is extended to magnitudes below 
those visible to the unaided eye. Each magni- 
tude compares with the one above it, as the light 
of two candles would compare with the light of 

Rutherfurd did not stop with mere photo- 
graphs. He realized very clearly the obvious 
truth that by making a picture of the sky we 
simply change the scene of our operations. 
Upon the photograph we can measure that which 
we might have studied directly in the heavens ; 
but so long as they remain unmeasured, celestial 



pictures have a potential value only. Locked 
within them may lie hidden some secret of our 
universe. But it will not come forth unsought. 
Patient effort must precede discovery, in pho- 
tography, as elsewhere in science. There is no 
royal road. Rutherfurd devised an elaborate 
measuring-machine in which his photographs 
could be examined under the microscope with 
the most minute exactness. With this machine 
he measured a large number of his pictures ; and 
it has been shown quite recently that the results 
obtained from them are comparable in accuracy 
with those coming from the most highly ac- 
credited methods of direct eye-observation. 

And photographs are far superior in ease of ma- 
nipulation. Convenient day-observing under the 
microscope in a comfortable astronomical labora- 
tory is substituted for all the discomforts of a 
midnight vigil under the stars. The work of 
measurement can proceed in all weathers, whereas 
formerly it was limited strictly to perfectly clear 
nights. Lastly, the negatives form a permanent 
record, to which we can always return to correct 
errors or re-examine doubtful points. 



Rutherfurd's stellar work extended down to 
about 1877, and included especially parallax de- 
terminations and the photography of star-clusters. 
Each of these subjects is receiving close attention 
from later investigators, and, therefore, merits 
brief mention here. Stellar parallax is in one 
sense but another name for stellar distance. Its 
measurement has been one of the important 
problems of astronomy for centuries, ever since 
men recognized that the Copernican theory of 
our universe requires the determination of stellar 
distances for its complete demonstration. 

If the earth is swinging around the sun once a 
year in a mighty path or orbit, there must be 
changes of its position in space comparable in size 
with the orbit itself. And the stars ought to shift 
their apparent places on the sky to correspond 
with these changes in the terrestrial observer's 
position. The phenomenon is analogous to what 
occurs when we look out of a room, first througli 
one window, and then through another. Any 
object on the opposite side of the street will be 
seen in a changed direction, on account of the 
observer's having shifted his position from one 



window to the other. If the object seemed to 
be due north when seen from the first window, 
it will, perhaps, appear a little east of north from 
the other. But this change of direction will be 
comparatively small, if the object under observa- 
tion is very far away, in comparison with the dis- 
tance between the two windows. 

This is what occurs with the stars. The earth's 
orbit, vast as it is, shrinks into almost absolute 
insignificance when compared with the profound 
distances by which we are sundered from even 
the nearest fixed stars. Consequently, the shift- 
ing of their positions is also very small — so 
small as to be near the extreme limit separating 
that which is measurable from that which is be- 
yond human ken. 

Photography lends itself most readily to a 
study of this matter. Suppose a certain star is 
suspected of "having a parallax." In other 
words, we have reason to believe it near enough 
to admit of a successful measurement of distance. 
Perhaps it is a very bright star ; and, other 
things being equal, it is probably fair to assume 
that brightness signifies nearness. And astrono- 



mers have certain other indications of proximity 
that guide them in the selection of proper objects 
for investigation, though such evidence, of course, 
never takes the place of actual measurement. 

The star under examination is sure to have near 
it on the sky a number of stars so very small 
that we may safely take them to be immeasurably 
far away. The parallax star is among them, 
but not of them. We see it projected upon the 
background of the heavens, though it may in 
reality be quite near us, astronomically speaking. 
If this is really so, and the star, therefore, subject 
to the slight parallactic shifting already men- 
tioned, we can detect it by noting the suspected 
star's position among the surrounding small 
stars. For these, being immeasurably remote, 
will remain unchanged, within the limits of our 
powers of observation, and thus serve as points 
of reference for marking the apparent shifting of 
the brighter star we are actually considering. 

We have merely to photograph the region at 
various seasons of the year. Careful examina- 
tion of the photographs under the microscope 

will then enable us to measure the slightest dis- 



placement of the parallax star. From these 
measures, by a process of calculation, astrono- 
mers can then obtain the star's distance. It will 
not become known in miles ; we shall only ascer- 
tain how many times the distance between the 
earth and sun would have to be laid down like a 
measuring-rod, in order to cover the space sepa- 
rating us from the star : and the subsequent 
evaluation of this distance " earth to sun " in 
miles is another important problem in whose so- 
lution photography promises to be most useful. 

The above method of measuring stellar distance 
is, of course, subject to whatever slight uncertainty 
arises from the assumption that the small stars 
used for comparison are themselves beyond the 
possibility of parallactic shifting. But astron- 
omy possesses no better method. Moreover, 
the number of small stars used in this way is, of 
course, much larger in photography than it ever 
can be in visual work. In the former process, 
all surrounding stars can be photographed at 
once ; in the latter each star must be measured 
separately, and daylight soon intervenes to im- 
pose a limit on numbers. Usually only two can 



be used ; so that here photography has a most 
important advantage. It minimizes the chance 
of our parallax being rendered erroneous, by the 
stars of comparison not being really infinitely 
remote. This might happen, perhaps, in the 
case of one or two ; but with an average result 
from a large number we know it to be practically 

Cluster work is not altogether unlike " paral- 
lax hunting " in its preliminary stage of securing 
the photographic observations. The object is to 
obtain an absolutely faithful picture of a star 
group, just as it exists in the sky. We have 
every reason to suppose that a very large num- 
ber of stars condensed into one small spot upon 
the heavens means something more than chance 
aggregation. The Pleiades group (page lo) con- 
tains thousands of massive stars, doubtless held 
together by the force of their mutual gravita- 
tional attraction. If this be true, there must be 
complex orbital motion in the cluster ; and, as 
time goes on, we should actually see the sepa- 
rate components change their relative positions, 

as it were, before our eyes. The details of such 



motion upon the great scale of cosmic space offer 
one of the many problems that make astronomy 
the grandest of human sciences. 

We have said that time must pass before we 
can see these things ; there may be centuries of 
waiting. But one way exists to hurry on the 
perfection of our knowledge ; we must increase 
the precision of observations. Motions that 
would need the growth of centuries to become 
visible to the older astronomical appliances, 
might yield in a few decades to more delicate 
observational processes. Here photography is 
most promising. Having once obtained a sur- 
passingly accurate picture of a star-cluster, we 
can subject it easily to precise microscopic meas- 
urement. The same operations repeated at a 
later date will enable us to compare the two 
series of measures, and thus ascertain the mo- 
tions that may have occurred in the interval. 
The Rutherfurd photographs furnish a veritable 
mine of information in researches of this kind ; 
for they antedate all other celestial photographs 
of precision by at least a quarter-century, and 
bring just so much nearer the time when definite 



knowledge shall replace information based on 
reasoning from probabilities. 

Rutherfurd's methods showed the advantages 
of photography as applied to individual star- 
clusters. It required only the attention of some 
astronomer disposing of large observational facili- 
ties, and accustomed to operations upon a great 
scale, to apply similar methods throughout the 
whole heavens. In the year i88a a bright 
comet was very conspicuous in the southern 
heavens. It was extensively observed from the 
southern hemisphere, and especially at the British 
Royal Observatory at the Cape of Good Hope, 

Gill, director of that institution, conceived the 
idea that this comet might be bright enough 
to photograph. At that time, comet photogra- 
phy had been attempted but little, if at all, and it 
was by no means sure that the experiment would 
be successful. Nor was Gill well acquainted with 
the work of Rutherfurd ; for the best results of 
that astronomer had lain dormant many years. 
He was one of those men with whom personal 
modesty amounts to a fault. Loath to put him- 
self forward in any way, and disliking to rush 



into print, Rutherfurd had given but little pub- 
licity to his work. This peculiarity has, doubt- 
less, delayed his just reputation ; but he will lose 
nothing in the end from a brief postponement. 
Gill must, however, be credited with more pene- 
tration than would be his due if Rutherfurd had 
made it possible for others to know that he had 
anticipated many of the newer ideas. 

However this may be, the comet was photo- 
graphed with the help of a local portrait photog- 
rapher named Allis. When Gill and Allis fast- 
ened a simple portrait camera belonging to the 
latter upon the tube of one of the Cape tele- 
scopes, and pointed it at the great comet, they 
little thought the experiment would lead to one 
of the greatest astronomical works ever at- 
tempted by men. Yet this was destined to oc- 
cur. The negative they obtained showed an 
excellent picture of the comet; but what was 
more important for the future of sidereal astron- 
omy, it was also quite thickly dotted with little 
black points corresponding to stars. The extra- 
ordinary ease with which the whole heavens 
could be thus charted photographically was 


brought home to Gill as never before. It was 
this comet picture that interested him in the ap- 
plication of photography to star-charting ; and 
without his interest the now famous astro-photo- 
graphic catalogue of the heavens would probably 
never have been made. 

After considerable preliminary correspondence, 
a congress of astronomers was finally called to 
meet at Paris in 1887. Representatives of the 
principal observatories and civilized governments 
were present. They decided that the end of the 
nineteenth century should see the making of a 
great catalogue of all the stars in the sky, upon a 
scale of completeness and precision surpassing 
anything previously attempted. It is impossible 
to exaggerate the importance of such a work ; 
for upon our star-catalogues depends ultimately 
the entire structure of astronomical science. 

The work was far too vast for the powers of 
any observatory alone. Therefore, the whole 
sky, from pole to pole, was divided into eighteen 
belts or zones of approximately equal area ; and 
each of these was assigned to a single observa- 
tory to be photographed. A series of telescopes 



was specially constructed, so that every part of 
the work should be done with the same type of 
instrument. As far as possible, an attempt was 
made to secure uniformity of methods, and par- 
ticularly a uniform scale of precision. To cover 
the entire sky upon the plan proposed no less 
than 44,108 negatives are required ; and most 
of these have now been finished. The further 
measurement of the pictures and the drawing up 
of a vast printed star-catalogue are also well un- 
der way. One of the participating observatories, 
that at Potsdam, Germany, has published the first 
volume of its part of the catalogue. It is esti- 
mated that this observatory alone will require 
twenty quarto volumes to contain merely the 
final results of its work on the catalogue. Alto- 
gether not less than two million stars will find a 
place in this, our latest directory of the heavens. 

Such wholesale methods of attacking problems 
of observational astronomy are particularly char- 
acteristic of photography. The great catalogue 
is, perhaps, the best illustration of this tendency ; 
but of scarcely smaller interest, though less im- 
portant in reality, is the photographic method of 



dealing with minor planets. We have already 
said (page 62) that in the space between the orbits 
of Mars and Jupiter several hundred small bodies 
are moving around the sun in ordinary planetary 
orbits. These bodies are called asteroids, or 
minor planets. The visual method of discover- 
ing unknown members of this group was pain- 
fully tedious ; but photography has changed 
matters completely, and has added immensely 
to our knowledge of the asteroids. 

Wolf, of Heidelberg, first made use of the 
new process for minor-planet discovery. His 
method is sufficiently ingenious to deserve brief 
mention again. A photograph of a suitable re- 
gion of the sky was made with an exposure last- 
ing two or three hours. Throughout all this 
time the instrument was manipulated so as to 
follow the motion of the heavens in the way we 
have already explained, so that each star would 
appear on the negative as a small, round, black 

But if a minor planet happened to be in the 
region covered by the plate, its photographic 

image would be very different. For the orbital 



motion of the planet about the sun would make 
it move a little among the stars even in the two 
or three hours during which the plate was ex- 
posed. This motion would be faithfully repro- 
duced in the picture, so that the planet would 
appear as a short curved line rather than a well- 
defined dot like a star. Thus the presence of 
such a line-image infallibly denotes an asteroid. 

Subsequent calculations are necessary to ascer- 
tain whether the object is a planet already known 
or a genuine new discovery. Wolf, and others 
using his method in recent years, have made im- 
mense additions to our catalogue of asteroids. 
Indeed, the matter was beginning to lose inter- 
est on account of the frequency and sameness 
of these discoveries, when the astronomical world 
was startled by the finding of the Planet of 1898. 
(Page 58.) 

On August 27, 1898, Witt, of Berlin, discov- 
ered the small body that bears the number 
"433 " in the list of minor planets, and has re- 
ceived the name Eros. Its important peculiar- 
ity consists in the exceptional position of the 

orbit. While all the other asteroids are farther 



from the sun than Mars, and less distant than 
Jupiter, Eros can pass within the orbit of the 
former. At times, therefore, it will approach 
our earth more closely than any other permanent 
member of the solar system, excepting our own 
moon. So it is, in a sense, our nearest neigh- 
bor ; and this fact alone makes it the most inter- 
esting of all the minor planets. The nineteenth 
century was opened by Piazzi's well-known dis- 
covery of the first of these bodies (page 59) ; it 
is, therefore, fitting that we should find the most 
important one at its close. We are almost cer- 
tain that it will be possible to make use of Eros 
to solve with unprecedented accuracy the most 
important problem in all astronomy. This is the 
determination of our earth's distance from the sun. 
When considering stellar parallax, we have seen 
how our observations enable us to measure some 
of the stars' distances in terms of the distance 
" earth to sun " as a unit. It is, indeed, the fun- 
damental unit for all astronomical measures, and 
its exact evaluation has always been considered 
the basal problem of astronomy. Astronomers 
know it as the problem of Solar Parallax. 



We shall not here enter into the somewhat 
intricate details of this subject, however interest- 
ing they may be. The problem offers diffi- 
culties somewhat analogous to those confronting 
a surveyor who has to determine the distance of 
some inaccessible terrestrial point. To do this, 
it is necessary first to measure a " base-line," as 
we call it. Then the measurement of angles 
with a theodolite will make it possible to deduce 
the required distance of the inaccessible point by 
a process of calculation. To insure accuracy, 
however, as every surveyor knows, the base-line 
must be made long enough ; and this is precise- 
ly what is impossible in the case of the solar 

For we are necessarily limited to marking 
out our base-line on the earth ; and the entire 
planet is too small to furnish one of really suffi- 
cient size. The best we can do is to use the dis- 
tance between two observatories situated, as near 
as may be, on opposite sides of the earth. But 
even this base is wofully small. However, the 
smallness loses some of its harmful effect if we 
operate upon a planet that is comparatively near 



us. We can measure such a planet's distance 
more accurately than any other ; and this being 
known, the solar distance can be computed by 
the aid of mathematical considerations based 
upon Newton's law of gravitation and observa- 
tional determinations of the planetary orbital 

Photography is by no means limited to inves- 
tigations in the older departments of astronom- 
ical observation. Its powerful arm has been 
stretched out to grasp as well the newer instru- 
ments of spectroscopic study. Here the sensi- 
tive plate has been substituted for the human 
eye with even greater relative advantage. The 
accurate microscopic measurement of difficult 
lines in stellar spectra was indeed possible by 
older methods ; but photography has made it 
comparatively easy ; and, above all, has ren- 
dered practicable series of observations extensive 
enough in numbers to furnish statistical informa- 
tion of real value. Only in this way have we 
been able to determine whether the stars, in their 
varied and unknown orbits, are approaching us 
or moving farther away. Even the speed of this 


Solar Corona. Total Eclipse. 
Photographed by Campbell, January 22, 1898 ; Jeur, India. 


approach or recession has become measurable, 
and has been evaluated in the case of many in- 
dividual stars. (See page 21.) 

The subject of solar physics has become a ver- 
itable department of astronomy in the hands of 
photographic investigators. Ingenious spectro- 
photographic methods have been devised, where- 
by we have secured pictures of the sun from 
which we have learned much that must have 
remained forever unknown to older methods. 

Especially useful has photography proved itself 
in the observation of total solar eclipses. It is 
only when the sun's bright disk is completely 
obscured by the interposed moon that we can see 
the faintly luminous structure of the solar co- 
rona, that great appendage of our sun, whose 
exact nature is still unexplained. Only during 
the few minutes of total eclipse in each century 
can we look upon it ; and keen is the interest of 
astronomers when those few minutes occur. But 
it is found that eye observations made in hur- 
ried excitement have comparatively little value. 
Half a dozen persons might make drawings of 
the corona during the same eclipse, yet they 



would differ so much from one another as to 
leave the true outline very much in doubt. But 
with photography we can obtain a really correct 
picture whose details can be studied and dis- 
cussed subsequently at leisure. 

If we were asked to sum up in one word what 
photography has accomplished, we should say 
that observational astronomy has been revolu- 
tionized. There is to-day scarcely an instru- 
ment of precision in which the sensitive plate 
has not been substituted for the human eye ; 
scarcely an inquiry possible to the older method 
which cannot now be undertaken upon a grander 
scale. Novel investigations formerly not even 
possible are now entirely practicable by photog- 
raphy ; and the end is not yet. Valuable as are 
the achievements already consummated, photog- 
raphy is richest in its promise for the future. 
Astronomy has been called the " perfect sci- 
ence " ; it is safe to predict that the next gen- 
eration will wonder that the knowledge we have 
to-day should ever have received so proud a 


The question is often asked, "What is the 
practical use of astronomy ? " We know, of 
course, that men would profit greatly from a study 
of that science, even if it could not be turned to 
any immediate bread-and-butter use ; for astron- 
omy is essentially the science of big things, and it 
makes men bigger to fix their minds on problems 
that deal with vast distances and seemingly end- 
less periods of time. No one can look upon the 
quietly shining stars without being impressed by 
the thought of how they burned — then as now — 
before he himself was born, and so shall continue 
after he has passed away — aye, even after his lat- 
est descendants shall have vanished from the 
earth. Of all the sciences, astronomy is at once 
the most beautiful poetically, and yet the one 
offering the grandest and most difficult problems 
to the intellect. A study of these problems has 


ever been a labor of love to the greatest minds ; 
their solution has been counted justly among 
man's loftiest achievements. 

And yet of all the difficult and abstruse sciences, 
astronomy is, perhaps, the one that comes into the 
ordinary practical daily life of the people more 
definitely and frequently than any other. There 
exist at least three things we owe to astronomy 
that must be regarded as quite indispensable, 
from a purely practical point of view. In the first 
place, let us consider the maps in a work on geog- 
raphy. How many people ever think to ask 
how these maps are made ? It is true that the 
ordinary processes of the surveyor would enable 
us to draw a map showing the outhnes of a part of 
the earth's surface. Even the locations of towns 
and rivers might be marked in this way. But one 
of the most important things of all could not be 
added without the aid of astronomical observa- 
tions. The latitude and longitude lines, which are 
essential to show the relation of the map to the 
rest of the earth, we owe to astronomy. The lon- 
gitude lines, particularly, as we shall see farther on, 
play a most important part in the subject of time. 



The second indispensable application of as- 
tronomy to ordinary business affairs relates to the 
subject of navigation. How do ships find their 
way across the ocean ? There are no permanent 
marks on the sea, as there are on the land, by 
which the navigator can guide his course. Never- 
theless, seamen know their path over the track- 
less ocean with a certainty as unerring as would 
be possible on shore ; and it is all done by the 
help of astronomy. The navigator's observa- 
tions of the sun are astronomical observations ; 
the tables he uses in calculating his observations — 
the tables that tell him just where he is and in what 
direction he must go — are astronomical tables. 
Indeed, it is not too much to say that without 
astronomy there could be no safe ocean navigation. 

But the third application of astronomy is of 
still greater importance in our daily hfe — the fur- 
nishing of correct time standards for all sorts of 
purposes. It is to this practical use of astro- 
nomical science that we would direct particular at- 
tention. Few persons ever think of the compli- 
cated machinery that must be put in motion in 
order to set a clock. A man forgets some even- 



ing to wind his watch at the accustomed hour. 
The next morning he finds it run down. It 
must be re-set. Most people simply go to the 
nearest clock, or ask some friend for the time, so 
as to start the watch correctly. More careful per- 
sons, perhaps, visit the jeweller's and take the 
time from his "regulator." But the regulator 
itself needs to be regulated. After all, it is noth- 
ing more than any other clock, except that greater 
care has been taken in the mechanical construction 
and arrangement of its various parts. Yet it is 
but a machine built by human hands, and, like 
all human works, it is necessarily imperfect. No 
matter how well it has been constructed, it will 
not run with perfectly rigid accuracy. Every day 
there will be a variation from the true time by a 
small amount, and in the course of days or weeks 
the accumulation of these successive small amounts 
will lead to a total of quite appreciable size. 

Just as the ordinary citizen looks to the jewel- 
ler's regulator to correct his watch, so the jeweller 
applies to the astronomer for the correction of his 
regulator. Ever since the dawn of astronomy, in 

the earliest ages of which we have any record, the 



principal duty of the astronomer has been the fur- 
nishing of accurate time to the people. We shall 
not here enter into a detailed account, however in- 
teresting it would be, of the gradual development 
by which the very perfect system at present in 
use has been reached ; but shall content ourselves 
with a description of the methods now employed 
in nearly all the civilized countries of the world. 

In the first place, every observatory is, of course, 
provided with what is known as an astronomical 
clock. This instrument, from the astronomer's 
point of view, is something very different from 
the ordinary popular idea. To the average per- 
son an astronomical clock is a complicated and 
elaborate affair, giving the date, day of the week, 
phases of the moon, and other miscellaneous in- 
formation. But in reality the astronomer wants 
none of these things. His one and only require- 
ment is that the clock shall keep as near uniform 
time as may be possible to a machine constructed 
by human hands. No expense is spared in mak- 
ing the standard clock for an observatory. Real 
artists in mechanical construction — men who have 
attained a world-wide celebrity for delicate skill 



in fashioning the parts of a dock — such are the 
astronomer's clock-makers. 

To increase precision of motion in the train of 
wheels, it is necessary that the mechanism be as 
simple as possible. For this reason all complica- 
tions of date, etc., are left out. We have even 
abandoned the usual convenient plan of having 
the hour and minute hands mounted at the same 
centre ; for this kind of mounting makes neces- 
sary a slightly more intricate form of wheelwork. 
The astronomer's clock usually has the centres 
of the second hand, minute hand, and hour hand 
in a straight line, and equally distant from each 
other. Each hand has its own dial ; all drawn, 
of course, upon the same clock-face. 

Even after such a clock has been made as ac- 
curately as possible, it will, nevertheless, not give 
the very best performance unless it is taken care 
of properly. It is necessary to mount it very 
firmly indeed. It should not be fastened to an 
ordinary wall, but a strong pier of masonry or 
brick must be built for it on a very solid founda- 
tion. Moreover, this pier is best placed under- 
ground in a cellar, so that the temperature of the 



clock can be kept nearly uniform all the year 
round ; for we find that clocks do not run quite 
the same in hot weather as they do in cold. 
Makers have, indeed, tried to guard against this 
effect of temperature, by ingenious mechanical 
contrivances. But these are never quite perfect 
in their action, and it is best not to test them too 
severely by exposing the clock to sharp changes 
of heat and cold. 

Another thing affecting the going of fine 
clocks, strange as it may seem, is the variation of 
barometric pressure. There is a slight but no- 
ticeable difference in their running when the 
barometer is high and when it is low. To pre- 
vent this, some of our best clocks have been en- 
closed in air-tight cases, so that outside barometric 
changes may not be felt in the least by the clock 

But even after all this has been accomplished, 
and the astronomer is in possession of a clock that 
may be called a masterpiece of mechanical con- 
struction, he is not any better off than was the 
jeweller with his regulator. After all, even the 
astronomical clock needs to be set, and its error 



must be determined from time to time. A 
final appeal must then be had to astronomical 
observations. The clock must be set by the 
stars and sun. For this purpose the astronomer 
uses an instrument called a " transit." This is 
simply a telescope of moderate size, possibly five 
or six feet long, and firmly attached to an axis at 
right angles to the tube of the telescope. 

This axis is supported horizontally in such a 
way that it points as nearly as may be exactly 
east and west. The telescope itself being square 
with the axis, always points in a north-and-south 
direction. It is possible to rotate the telescope 
about its axis so as to reach all parts of the sky 
that are directly north or south of the observa- 
tory. In the field of view of the telescope cer- 
tain very fine threads are mounted so as to form 
a little cross. As the telescope is rotated this 
cross traces out, as it were, a great circle on the 
sky ; and this great circle is called the astronomi- 
cal meridian. 

Now we are in possession of certain star-tables, 

computed from the combined observations of 

astronomers in the last 150 years. These tables 



tell us the exact moment of time when any star 
is on the meridian. To discover, therefore, 
whether our clock is right on any given night, it 
is merely necessary to watch a star with the tele- 
scope, and note the exact instant by the clock 
when it reaches the little cross in the field of 
view. Knowing from the astronomical tables the 
time when the star ought to have been on the 
meridian, and having observed the clock time 
when it is actually there, the difference is, of 
course, the error of the clock. The result can 
be checked by observations of other stars, and 
the slight personal errors of observation can be 
rendered harmless by taking the mean from sev- 
eral stars. By an hour's work on a fine night it 
is possible to fix the clock error quite easily 
within the one-twentieth part of a second. 

We have not space to enter into the interest- 
ing details of the methods by which the astro- 
nomical transit is accurately set in the right 
position, and how any slight residual error in 
its setting can be eliminated from our results by 
certain processes of computation. It must suffice 

to say that practically all time determinations in 



the observatory depend substantially upon the 
procedure outlined above. 

The observatory clock having been once set 
right by observations of the sky, its error can be 
re-determined every few days quite easily. Thus 
even the small irregularities of its nearly perfect 
mechanism can be prevented from accumulating 
until they might reach a harmful magnitude. 
But we obtain in this way only a correct standard 
of time within the observatory itself. How can 
this be made available for the general public ? 
The problem is quite simple with the aid of the 
electric telegraph. We shall give a brief account 
of the methods now in use in New York City, 
and these may be taken as essentially representa- 
tive of those employed elsewhere. 

Every day, at noon precisely, an electric signal 
is sent out by the United States Naval Observa- 
tory in Washington. The signal is regulated by 
the standard clock of the observatory, of course 
taking account of star observations made on the 
next preceding fine night. This signal is re- 
ceived in the central New York office of the tele- 
graph company, where it is used to keep correct 


a very fine clock, which may be called the time 
standard of the telegraph company. This clock, 
in turn, has automatic electric connections, by 
means of which it is made to send out signals 
over what are called " time wires " that go all 
over the city. Jewellers, and others who desire 
correct time, can arrange to have a small electric 
sounder in their offices connected with the time 
wires. Thus the ticks of the telegraph com- 
pany's standard clock are repeated automatically 
in the jeweller's shop, and used for controlling 
the exactness of his regulator. This, in brief, is 
the method by which the astronomer's careful 
determination of correct time is transferred and 
distributed to the people at large. 

Having thus outlined the manner of obtaining 
and distributing correct time, we shall now consider 
the question of time differences between different 
places on the earth. This is a matter which many 
persons find most perplexing, and yet it is essential- 
ly quite simple in principle. Travellers, of course, 
are well acquainted with the fact that their watches 
often need to be reset when they arrive at their 
destination. Yet few ever stop to ask the cause. 


Let us consider for a moment our method of 
measuring time. We go by the sun. If we leave 
out of account some small irregularities of the sun's 
motion that are of no consequence for our present 
purpose, we may lay down this fundamental prin- 
ciple : When the sun reaches its highest position 
in the sky it is twelve o'clock or noon. 

The sun, as everyone knows, rises each morn- 
ing in the east, slowly goes up higher and higher 
in the sky, and at last begins to descend again 
toward the west. But it is clear that as the sun 
travels from east to west, it must pass over the 
eastern one of any two cities sooner than the 
western one. When it reaches its greatest height 
over a western city it has, therefore, already passed 
its greatest height over an eastern one. In other 
words, when it is noon, or twelve o'clock, in the 
western city, it is already after noon in the eastern 
city. This is the simple and evident cause of 
time differences in different parts of the country. 
Of any two places the eastern one always has later 
time than the western. When we consider the 
matter in this way there is not the slightest diffi- 
culty in understanding how time differences arise. 



They will, of course, be greatest for places that 
are very far apart in an east-and-west direction. 
And this brings us again to the subject of longi- 
tude, which, as we have already said, plays an 
important part in all questions relating to time ; 
for longitude is used to measure the distance in 
an east-and-west direction between different parts 
of the earth. 

If we consider the earth as a large ball we can 
imagine a series of great circles drawn on its surface 
and passing directly from the North Pole to the 
South Pole. Such a circle could be drawn through 
any point on the earth. If we imagine a pair of 
them drawn through two cities, such as New York 
and London, the longitude difference of these two 
cities is defined as the angle at the North Pole be- 
tween the two great circles in question. The size 
of this angle can be expressed in degrees. If we 
then wish to know the difference in time between 
New York and London in hours, we need only 
divide their longitude difference in degrees by the 
number 15. In this simple way we can get the 
time difference of any two places. We merely 
measure the longitude diflference on a map, and 



then divide by 15 to get the time difference. 
These time differences can sometimes become 
quite large. Indeed, for two places differing 1 80 
degrees in longitude, the time difference will evi- 
dently be no less than twelve hours. 

Most civilized nations have agreed informally 
to adopt some one city as the fundamental point 
from which all longitudes are to be counted. Up 
to the present we have considered only longitude 
differences ; but when we speak of the longitude 
of a city we mean its longitude difference from 
the place chosen by common consent as the ori- 
gin for measuring longitudes. The town almost 
universally used for this purpose is Greenwich, 
near London, England. Here is situated the 
British Royal Observatory, one of the oldest and 
most important institutions of its kind in the 
world. The great longitude circle passing through 
the centre of the astronomical transit at the Green- 
wich observatory is the fundamental longitude 
circle of the earth. The longitude of any other 
town is then simply the angle at the pole between 
the longitude circle through that town and the 

fundamental Greenwich one here described. 



Longitudes are counted both eastward and 
westward from Greenwich. Thus New York is 
in 74 degrees west longitude, while Berlin is in 
14 degrees east longitude. This has led to a 
rather curious state of affairs in those parts of the 
earth the longitudes of which are nearly 180 
degrees east or west. There are a number of 
islands in that part of the world, and if we imagine 
for a moment one whose longitude is just 1 80 de- 
grees, we shall have the following remarkable re- 
sult as to its time difference from Greenwich. 

We have seen that of any two places the eastern 
always has the later time. Now, since our imag- 
inary island is exactly 180 degrees from Green- 
wich, we can consider it as being either 1 80 de- 
grees east or 180 degrees west. But if we call it 
180 degrees east, its time will be twelve hours 
later than Greenwich, and if we call it 1 80 degrees 
west, its time will be twelve hours earlier than 
Greenwich. Evidently there will be a difference 
of just twenty-four hours, or one whole day, be- 
tween these two possible ways of reckoning its 
time. This circumstance has actually led to con- 
siderable confusion in some of the islands of the 



Pacific Ocean. The navigators who discovered 
the various islands naturally gave them the date 
which they brought from Europe. And as some 
of these navigators sailed eastward, around the 
Cape of Good Hope, and others westward, around 
Cape Horn, the dates they gave to the several 
islands diflFered by just one day. 

The state of affairs at the present time has 
been adjusted by a sort of informal agreement. 
An arbitrary line has been drawn on the map 
near the i8oth longitude circle, and it has been 
decided that the islands on the east side of this 
line shall count their longitudes west from Green- 
wich, and those west of the line shall count lon- 
gitude east from Greenwich. Thus Samoa is 
nearly 1 80 degrees west of Greenwich, while the 
Fiji Islands are nearly 180 degrees east. Yet the 
islands are very near each other, though the ar- 
bitrary line passes between them. As a result, 
when it is Sunday in Samoa it is Monday in the 
Fiji Islands. The arbitrary line described here 
is sometimes called the International Date-Line. 

It does not pass very near the Philippine 

Islands, which are situated in about 120 degrees 



east longitude, and, therefore, use a time about 
eight hours later than Greenwich. New York, 
being about 74 degrees west of Greenwich, is 
about five hours earlier in time. Consequently, 
as we may remark in passing, Philippine time is 
about thirteen hours later than New York time. 
Thus, five o'clock, Sunday morning. May ist, 
in Manila, would correspond to four o'clock, 
Saturday afternoon, April 30th, in New York. 

There is another kind of time which we shall ex- 
plain briefly — the so-called " standard," or railroad 
time, which came into general use in the United 
States some few years ago, and has since been 
generally adopted throughout the world. It re- 
quires but a few moments' consideration to see 
that the accidental situation of the different large 
cities in any country will cause their local times 
to differ by odd numbers of hours, minutes, and 
seconds. Thus a great deal of inconvenience 
has been caused in the past. For instance, a 
train might leave New York at a certain hour by 
New York time. It would then arrive in Buf- 
falo some hours later by New York time. But 

it would leave Buffalo by Buffalo time, which is 



quite different. Thus there would be a sort of 
jump in the time-table at Buffalo, and it would 
be a jump of an odd number of minutes. 

It would be different in different cities, and very 
hard to remember. Indeed, as each railway usu- 
ally ran its trains by the time used in the princi- 
pal city along its line, it might happen that three 
or four different railroad times would be used in 
a single city where several roads met. This has 
all been avoided by introducing the standard 
time system. According to this the whole coun- 
try is divided into a series of time zones, fifteen 
degrees wide, and so arranged that the middle 
line of each zone falls at a point whose longitude 
from Greenwich is 60, 75, 90, 105, or 120 de- 
grees. The times at these middle lines are, 
therefore, earlier than Greenwich time by an 
even number of hours. Thus, for instance, the 
75-degree line is just five even hours earlier than 
Greenwich time. All cities simply use the time 
of the nearest one of these special lines. 

This does not result in doing away with time 
differences altogether — that would, of course, be 
impossible in the nature of things — but for the 



complicated odd differences in hours and minutes, 
we have substituted the infinitely simpler series 
of differences in even hours. The traveller from 
Chicago to New York can reset his watch by 
putting it just one hour later on his arrival — the 
minute hand is kept unchanged, and no New 
York timepiece need be consulted to set the 
watch right on arriving. There can be no doubt 
that this standard-time system must be consid- 
ered one of the most important contributions of 
astronomical science to the convenience of man. 

Its value has received the widest recognition, 
and its use has now extended to almost all civil- 
ized countries — France is the only nation of im- 
portance still remaining outside the time-zone 
system. In the following table we give the 
standard time of the various parts of the earth as 
compared with Greenwich, together with the date 
of adopting the new time system. It will be 
noticed that in certain cases even half-hours have 
been employed to separate the time-zones, in- 
stead of even hours as used in the United States. 




When it is Noon 

Date of Adopting 

at Greenwich 


Standard Time 

it is 



Great Britain. 


May, 1892. 


May, 1892. 


, January, 1 90 1 . 

I P.M. 


April, 1893. 


November, 1893. 


j January, 1894. 


; June, 1894. 


January, 1895. 

Austria (railways). 


1.30 P.M. 

Cape Colony. 


Orange River Colony. 




2 P.M. 


Turkey (railwayi). 

September, 1895. 


1 October, 1900. 

8 P.M. 

West Australia. 

February, 1895. 

9 P.M. 



9.30 P.M. 

South Australia. 

May, 1899. 

10 P.M. 


February, 1895. 

New South Wales. 

February, 1895. 


February, 1895. 

II P.M. 

New Zealand. 

In the United States and Canada it is 

4 A.M. by Pacific Time when it is Noon at Greenwich. 

5 A.M. " Mountain " " " «< " " 

6 A.M. " Central " " " " " " 

7 A.M. " Eastern " " " " " " 

8 «.M. " Colonial " " " " " " 



Students of geology have been puzzled for 
many years by traces remaining from the period 
when a large part of the earth was covered with a 
heavy cap of ice. These shreds of evidence all 
seem to point to the conclusion that the centre of 
the ice-covered region was quite far away from 
the present position of the north pole of the earth. 
If we are to regard the pole as very near the 
point of greatest cold, it becomes a matter of 
much interest to examine whether the pole has 
always occupied its present position, or whether 
it has been subject to slow changes of place upon 
the earth's surface. Therefore, the geologists 
have appealed to astronomers to discover whether 
they are in possession of any observational evi- 
dence tending to show that the pole is in motion. 

Now we may say at once that astronomical re- 
search has not as yet revealed the evidence thus 

expected. Astronomy has been unable to come 



to the rescue of geological theory. From about 
the year 1750, which saw the beginning of precise 
observation in the modern sense, down to very 
recent times, astronomers were compelled to deny 
the possibility of any appreciable motion of the 
pole. Observational processes, it is true, fur- 
nished slightly divergent pole positions from time 
to time. Yet these discrepancies were always so 
minute as to be indistinguishable from those slight 
personal errors that are ever inseparable from re- 
sults obtained by the fallible human eye. 

But in the last few years improved methods of 
observation, coupled with extreme diligence in 
their application by astronomers generally, have 
brought to light a certain small motion of the 
pole which had never before been demonstrated 
in a reliable way. This motion, it is true, is not 
of the character demanded by geological theory , 
for the geologists had been led to expect a motion 
which would be continuous in the same direction, 
no matter how slow might be its annual amount ; 
for the vast extent of geologic time would give 
even the slowest of motions an opportunity to 

produce large effects, provided its results could 



be continuously cumulative. Given time enough, 
and the pole might move anywhere on the earth, 
no matter how slow might be its tortoise speed. 

But the small motion we have discovered is 
neither cumulative nor continuous in one direc- 
tion. It is what we call a periodic motion, the 
pole swinging now to one side, and now to the 
other, of its mean or average position. Thus 
this new discovery cannot be said to unravel the 
mysterious puzzle of the geologists. Yet it is 
not without the keenest interest, even from their 
point of view; for the proof of any form of 
motion in a pole previously supposed to be abso- 
lutely at rest may mean everything. No man 
can say what results will be revealed by the fur- 
ther observations now being continued with great 

In the first place, it is important to explain that 
any such motions as we have under considera- 
tion will show themselves to ordinary observa- 
tional processes principally in the form of changes 
of terrestrial latitudes. Let us imagine a pair of 
straight lines passing through the centre of the 
earth and terminating, one at the observer's sta- 



tion on the earth's surface, and the other at that 
point of the equator which is nearest the observer. 
Then, according to the ordinary definition of lat- 
itude, the angle between these two imaginary lines 
is called the latitude of the point of observation. 
Now we know, of course, that the equator is 
everywhere just 90 degrees from the pole. Con- 
sequently, if the pole is subject to any motion at 
all, the equator must also partake of the n\otion. 

Thus the angle between our two imaginary lines 
will be affected directly by polar movement, and 
the latitude obtained by astronomical observation 
will be subject to quite similar changes. To clear 
up the whole question, so far as this can be done 
by the gathering of observational evidence, it is 
only necessary to keep up a continual series of 
latitude determinations at several observatories. 
These determinations should show small varia- 
tions similar in magnitude to the wabblings of the 

Let us now consider for a moment what is 
meant by the axis of the earth. It has long been 
known that the planet has in general the shape of 
a ball or sphere. That this is so can be seen at 



once from the way ships at sea disappear at the 
horizon. As they go farther and farther from us, 
we first lose sight of the hull, and then slowly 
and gradually the spars and sails seem to sink 
down into the ocean. This proves that the 
earth's surface is curved. That it is more or less 
like a sphere is evident from the fact that it always 
casts a round shadow in eclipses. Sometimes the 
earth passes between the sun and eclipsed moon. 
Then we see the earth's black shadow projected 
on the moon, which would otherwise be quite 
bright. This shadow has been observed In a very 
large number of such eclipses, and it has always 
been found to have a circular edge. 

While, therefore, the earth is nearly a round 
ball, it must not be supposed that it is exactly 
spherical in form. We may disregard the small 
irregularities of its surface, for even the greatest 
mountains are insignificant in height when com- 
pared with the entire diameter of the earth itself. 
But even leaving these out of account, the earth is 
not perfectly spherical. We can describe it best 
as a flattened sphere. It is as though one were 
to press a round rubber ball between two smooth 



boards. It would be flattened at the top and 
bottom and bulged out in the middle. This is 
the shape of the earth. It is flattened at the 
poles and bulges out near the equator. The 
shortest straight line that can be drawn through 
the earth's centre and terminated by the flattened 
parts of its surface may be called the earth's axis 
of figure ; and the two points where this axis 
meets the surface are called the poles of figure. 

But the earth has another axis, called the axis 
of rotation. This is the one about which the 
planet turns once in a day, giving rise to the well- 
known phenomena called the rising and setting of 
sun, moon, and stars. For these motions of the 
heavenly bodies are really only apparent ones, 
caused by an actual motion of the observer on 
the earth. The observer turns with the earth on 
its axis, and is thus carried past the sun and stars. 

This daily turning of the earth, then, takes 
place about the axis of rotation. Now, it so hap- 
pens that all kinds of astronomical observations 
for the determination of latitude lead to values 
based on the rotation axis of the earth, and not 

on its axis of figure. We have seen how the 



earth's equator, from which we count our lati- 
tudes, is everywhere 90 degrees distant from the 
pole. But this pole is the pole of rotation, or 
the point at which the rotation axis pierces the 
earth's surface. It is not the pole of figure. 

It is clear that the latitude of any observa- 
tory will remain constant only if the pole of 
figure and the rotation pole maintain absolutely 
the same positions relatively one to the other. 
These two poles are actually very near together ; 
indeed, it was supposed for a very long time that 
they were absolutely coincident, so that there could 
not be any variations of latitude. But it now 
appears that they are separated slightly. 

Strange to say, one of them is revolving 
about the other in a little curve. The pole of 
figure is travelling around the pole of rotation. 
The distance between them varies a little, never 
becoming greater than about fifty feet, and it 
takes about fourteen months to complete a revo- 
lution. There are some slight irregularities in 
the motion, but, in the main, it takes place in the 
manner here stated. In consequence of this ro- 
tation of the one pole about the other, the pole 



of figure is now on one side of the rotation pole 
and now on the opposite side, but it never travels 
continuously in one direction. Thus, as we have 
already seen, the sort of continuous motion re- 
quired to explain the observed geological phe- 
nomena has not yet been found by astronomers. 

Observations for the study of latitude varia- 
tions have been made very extensively within 
recent years both in Europe and the United 
States. It has been found practically most ad- 
vantageous to carry out simultaneous series of 
observations at two observatories situated in 
widely different parts of the earth, but having 
very nearly the same latitude. It is then pos- 
sible to employ the same stars for observation in 
both places, whereas it would be necessary to 
use different sets of stars if there were much 
difference in the latitudes. 

There is a special advantage in using the same 
stars in both places. We can then determine 
the small difference in latitude between the two 
participating observatories in a manner which will 
make it quite free from any uncertainty in our 
knowledge of the positions on the sky of the 



stars observed ; for, strange as it may seem, our 
star-catalogues do not contain absolutely accurate 
numbers. Like all other data depending on 
fallible human observation, they are affected with 
small errors. But if we can determine simply 
the difference in latitude of the two observatories, 
we can discover from its variation the path in 
which the pole is moving. If, for instance, the 
observatories are separated by one-quarter the 
circumference of the globe, the pole will be mov- 
ing directly toward one of them, when it is not 
changing its distance from the other one at all. 

This method was used for seven years with good 
effect at the observatories of Columbia Univer- 
sity in New York, and the Royal Observatory at 
Naples, Italy. For obtaining its most complete 
advantages it is, of course, better to establish sev- 
eral observing stations on about the same parallel 
of latitude. This was done in 1899 by the Inter- 
national Geodetic Association. Two stations are in 
the United States, one in Japan, and one in Sicily. 
We can, therefore, hope confidently that our knowl- 
edge as to the puzzling problem of polar motion 

will soon receive very material advancement. 



The death of James E. Keeler, Director of the 
Lick Observatory, in California (p. 3a), recalls 
to mind one of the most interesting and signifi- 
cant of later advances in astronomical science. 
Only seven years have elapsed since Keeler made 
the remarkable spectroscopic observations which 
gave for the first time an ocular demonstration of 
the true character of those mysterious luminous 
rings surrounding the brilliant planet Saturn. 
His results have not yet been made sufficiently 
accessible to the public at large, nor have they 
been generally valued at their true worth. We 
consider this work of Keeler's interesting, be- 
cause the problem of the rings has been a classic 
one for many generations ; and we have been 
particular, also, to call it significant, because it is 
pregnant with the possibilities of newer methods 
of spectroscopic research, applied in the older 

departments of observational astronomy. 



The troubles of astronomers with the rings 
began with the invention of the telescope itself 
They date back to 1610, when Galileo first 
turned his new instrument to the heavens (p. 49). 
It may be imagined easily that the bright planet 
Saturn was among the very first objects scruti- 
nized by him. His " powerful " instrument 
magnified only about thirty times, and was, 
doubtless, much inferior to our pocket telescopes 
of to-day. But it showed, at all events, that 
something was wrong with Saturn. Galileo put 
it, "UlUmarn planetam tergeminam observavi" 
(" I have observed the furthest planet to be 
triple " ). 

It is easy to understand now how Galileo's 
eyes deceived him. For a round luminous ball 
like Saturn, surrounded by a thin flat ring seen 
nearly edgewise, really looks as if it had two lit- 
tle attached appendages. Strange, indeed, it is 
to-day to read a scientific book so old that the 
planet Saturn could be called the "furthest" 
planet. But it was the outermost known in 
Galileo's day, and for nearly two centuries after- 
ward. Not until 1 78 1 did William Herschel 



discover Uranus (p. 59) ; and Neptune was not 
disclosed by the marvellous mathematical percep- 
tion of Le Verrier until 1846 (p. 61). 

Galileo's further observations of Saturn both- 
ered him more and more. The planet's behav- 
ior became much worse as time went on. " Has 
Saturn devoured his children, according to the 
old legend ? " he inquired soon afterward ; for 
the changed positions of earth and planet in the 
course of their motions around the sun in their 
respective orbits had become such that the ring 
was seen quite edgewise, and was, therefore, per- 
fectly invisible to Galileo's " optic tube." The 
puzzle remained unsolved by Galileo ; it was left 
for another great man to find the true answer. 
Huygens, in 1656, first announced that the ring 
is a ring. 

The manner in which this announcement was 
made is characteristic of the time ; to-day it 
seems almost ludicrous. Huygens published a 
little pamphlet in 1656 called " De Saturni Luna 
Observatio Nova" or, " A New Observation of 
Saturn's Moon." He gave the explanation of 

what had been observed by himself and preced- 



ing astronomers in the form of a puzzle, or 
" logogriph." Here is what he had to say of 
the phenomenon in question : 

" aaaaaaa ccccc d eeeee g h iiiiiii 1111 mm 
nnnnnnnnn oooo pp q rr s ttttt uuuuu." 

It was not until 1659, three years later, in a 
book entitled " Systema Saiurnium," that Huy- 
gens rearranged the above letters in their proper 
order, giving the Latin sentence : 

" Annulo cingitur, tenui piano, nusquam co- 
haerente, ad eclipticam inclinato." Translated 
into English, this sentence informs us that the 
planet " is girdled with a thin, flat ring, nowhere 
touching Saturn, and inclined to the ecliptic " ! 

This was a perfectly correct and wonderfully 
sagacious explanation of those complex and exas- 
peratingly puzzling phenomena that had been 
too difficult for no less a person than Galileo 
himself. It was an explanation that explained. 
The reason for its preliminary announcement in 
the above manner must have been the following : 
Huygens was probably not quite sure of his 
ground in 1656, while three years afterward he 
had become quite certain. By the publication of 



the logogriph of 1656 he secured for himself the 
credit of what he had done. If any other as- 
tronomer had published the true explanation 
after 1656, Huygens could have proved his 
claim to priority by rearranging the letters of his 
puzzle. On the other hand, if further researches 
showed him that he was wrong, he would never 
have made known the true meaning of his logo- 
griph, and would thus have escaped the igno- 
miny of making an erroneous explanation. Thus, 
the method of announcement was comparable 
in ingenuity with the Huygenian explanation 

We are compelled to pass over briefly the en- 
tertaining history of subsequent observations of 
the ring, in order to explain the new work of 
Keeler and others. Cassini, about 1675, had 
been able to show that the ring was double ; that 
there are really two independent rings, with a 
distinct dark space between them. It was a case 
of wheels within wheels. To our own eminent 
countryman, W. C. Bond, of Cambridge, Mass., 
we owe the further discovery (Harvard College 

Observatory, November, 1850) of the third 



ring. This is also concentric with the other two, 
and interior to them, but difficult to observe, 
because of its much smaller luminosity. 

It is almost transparent, and the brilliant light 
of the planet's central ball is capable of shining 
directly through it. For this reason the inner 
ring is called the " gauze " or " crape " ring. If 
we add to the above details the fact that our 
modern large telescopes show slight irregularities 
in the surface of the rings, especially when seen 
edgewise, we have a brief statement of all that 
the telescope has been able to reveal to us since 
Galileo's time. 

But of far greater interest than the mere fact 
of their existence is the important cosmic ques- 
tion as to the constitution, structure, and, above 
all, durabiHty of the ring system. Astronomers 
often use the term " stability " with regard to 
celestial systems like the ring system of Saturn. 
By this they mean permanent durability. A 
system is stable if its various parts can continue 
in their present relationship to one another, with- 
out violating any of the known laws of astron- 
omy. Whenever we study any collection of 



celestial objects, and endeavor to explain their 
motions and peculiarities, we always seek some 
explanation not inconsistent with the continued 
existence of the phenomena in question. For 
this there is, perhaps, no sufficient philosophical 
basis. Probably much of the great celestial pro- 
cession is but a passing show, to be but for a 
moment in the endless vista of cosmic time. 

However this may be, we are bound to as- 
sume as a working theory that Saturn has always 
had these rings, and will always have them ; and 
it is for us to find out how this is possible. The 
problem has been attacked mathematically by 
various astronomers, including Laplace ; but no 
conclusive mathematical treatment was obtained 
until 1857, when James Clark-Maxwell proved 
in a masterly manner that the rings could be 
neither solid nor liquid. He showed, indeed, 
that they would not last if they were continuous 
bodies like the planets. A big solid wheel 
would inevitably be torn asunder by any slight 
disturbance, and then precipitated upon the plan- 
et's surface. Therefore, the rings must be com- 
posed of an immense number of small detached 



particles, revolving around Saturn in separate 
orbits, like so many tiny satellites. 

This mathematical theory of the ring system 
being once established, astronomers were more 
eager than ever to obtain a visual confirmation 
of it. We had, indeed, a sort of analogy in 
the assemblage of so-called " minor planets " 
(p. 64), which are known to be revolving around 
our sun in orbits situated between Mars and 
Jupiter. Some hundreds of these are known 
to exist, and probably there are countless others 
too small for us to see. Such a swarm of tiny 
particles of luminous matter would certainly 
give the impression of a continuous solid body, 
if seen from a distance comparable to that sepa- 
rating us from Saturn. But arguments founded 
on analogy are of comparatively little value. 

Astronomers need direct and conclusive tele- 
scopic evidence, and this was lacking until Keeler 
made his remarkable spectroscopic observation in 
1895. The spectroscope is a peculiar instru- 
ment, different in principle from any other used 
in astronomy ; we study distant objects with it 
by analyzing the light they send us, rather than 



by examining and measuring the details of their 
visible surfaces. The reader will recall that ac- 
cording to the modern undulatory theory, light 
consists simply of a series of waves. Now, the 
nature of waves is very far from being under- 
stood in the popular mind. Most people, for 
instance, think that the waves of ocean consist of 
great masses of water rolling along the surface. 

This notion doubtless arises from the behavior 
of waves when they break upon the shore, form- 
ing what we call surf. When a wave meets 
with an immovable body like a sand beach, the 
wave is broken, and the water really does roll 
upon the beach. But this is an exceptional case. 
Farther away from the shore, where the waves 
are unimpeded, they consist simply of particles 
of water moving straight up and down. None 
of the water is carried by mere wave-action away 
from the point over which it was situated at first. 

Tides or other causes may move the water, but 

not simple wave-motion alone. That this is so 

can be proved easily. If a chip of wood be 

thrown overboard from a ship at sea It wilL be 

seen to rise and fall a long time on the waves, 



but it will not move. Similarly, wind-waves are 
often quite conspicuous on a field of grain ; but 
they are caused by the individual grain particles 
moving up and down. The grain certainly can- 
not travel over the ground, since each particle is 
fast to its own stalk. 

But while the particles do not travel, the wave- 
disturbance does. At times it is transmitted to 
a considerable distance from the point where it 
was first set in motion. Thus, when a stone Is 
dropped into still water, the disturbance (though 
not the water) travels in ever-widening circles, 
until at last it becomes too feeble for us to per- 
ceive. Light is just such a travelling wave-dis- 
turbance. Beginning, perhaps, in some distant 
star, it travels through space, and finally the 
wave impinges on our eyes like the ocean-wave 
breaking on a sand beach. Such a light-wave 
affects the eye in some mysterious way. We 
call it " seeing." 

The spectroscope (p. 21) enables us to meas- 
ure and count the waves reaching us each second 
from any source of light. No matter how far 

away the origin of stellar light may be, the spec- 



troscope examines the character of that light, and 
tells us the number of waves set up every sec- 
ond. It is this characteristic of the instrument 
that has enabled us to make some of the most 
remarkable observations of modern times. If 
the distant star is approaching us in space, more 
light -waves per second will reach us than we 
should receive from the same star at rest. Thus 
if we find from the spectroscope that there are 
too many waves, we know that the star is com- 
ing nearer ; and if there are too few, we can 
conclude with equal certainty that the star is 

Keeler was able to apply the spectroscope in 
this way to the planet Saturn and to the ring 
system. The observations required dexterity 
and observational manipulative skill in a superla- 
tive degree. These Keeler had ; and this work 
of his will always rank as a classic observation. 
He found by examining the light-waves from 
opposite sides of the planet that the luminous 
ball rotated ; for one side was approaching us 
and the other receding. This observation was, 

of course, in accord with the known fact of Sat- 



urn's rotation on his axis. With regard to the 
rings, Keeler showed in the same way the ex- 
istence of an axial rotation, which appears not 
to have been satisfactorily proved before, strange 
as it may seem. But the crucial point estab- 
lished by his spectroscope was that the interior 
part of the rings rotates /aj/^r than the exterior. 

The velocity of rotation diminishes gradually 
from the inside to the outside. This fact is ab- 
solutely inconsistent with the motion of a solid 
ring ; but it fits in admirably with the theory of 
a ring comprised of a vast assemblage of small 
separate particles. Thus, for the first time, as- 
tronomy comes into possession of an observa- 
tional determination of the nature of Saturn's 
rings, and Galileo's puzzle is forever solved. 



Astronomical discoveries are always received 
by the public with keen interest. Every new 
fact read in the great open book of nature is writ- 
ten eagerly into the books of men. For there 
exists a strong curiosity to ascertain just how the 
greater world is built and governed ; and it must 
be admitted that astronomers have been able to 
satisfy that curiosity with no small measure of 
success. But it is seldom that we hear of the 
means by which the latest and most refined 
astronomical observations are effected. Popular 
imagination pictures the astronomer, as he doubt- 
less once was, an aged gentleman, usually having 
a long white beard, and spending entire nights 
staring at the sky through a telescope. 

But the facts to-day are very different. The 
working astronomer is an active man in the 
prime of life, often a young man. He wastes no 
time in star-gazing. His observations consist of 



exact measurements made in a precise, systemat- 
ic, and almost business-like manner. A night's 
" watch " at the telescope is seldom allowed to 
exceed about three hoursj since it is found that 
more continued exertions fatigue the eye and lead 
to less accurate results. To this, of course, there 
have been many notable exceptions, for endurance 
of sight, like any form of physical strength, differs 
greatly in different individuals. Astronomical re- 
search does not include " picking out " the con- 
stellations, and learning the Arabic names of in- 
dividual stars. These things are not without 
interest ; but they belong to astronomy's ancient 
history, and are of little value except to afford 
amusement and instruction to successive genera- 
tions of amateurs. 

Among the instruments for carefully planned 
measurements of precision the heliometer prob- 
ably takes first rank. It is at once the most 
exquisitely accurate in its results, and the most 
fatiguing to the observer, of all the varied ap- 
paratus employed by the astronomer. The prin- 
ciple upon which its construction depends is very 
peculiar, and applies to all telescopes, even or- 



dinary ones for terrestrial purposes. If part of a 
telescope lens be covered up with the hand, it 
will still be possible to see through the instru- 
ment. The glass lens at the end of the tube 
farthest from the observer's eye helps to magnify 
distant objects and make them seem nearer by 
gathering to a single point, or focus, a greater 
amount of their light than could be brought 
together by the far smaller lens in the unaided 

The telescope might very properly be likened 
to an enlarged eye, which can see more than we 
can, simply because it is bigger. If a telescope 
lens has a surface one hundred times as large as 
that of the lens in our eye, it will gather and bring 
to a focus one hundred times as much light from a 
distant object. Now, if any part of this telescope 
be covered, the remaining part will, nevertheless, 
gather and focus light just as though the whole 
lens were in action ; only, there will be less light 
collected at the focus within the tube. The 
small lens at the telescope's eye-end is simply a 
magnifier to help our eye examine the image of 
any distant object formed at the focus by the 



large lens at the farther end of the instrument. 
For of this simple character is the operation of 
any telescope : the large glass lens at one end 
collects a distant planet's light, and brings it to a 
focus near the other end of the tube, where it 
forms a tiny picture of the planet, which, in 
turn, is examined with the little magnifier at 
the eye-end. 

Having arrived at the fundamental principle 
that part of a lens will act in a manner similar to 
a whole one, it is easy to explain the construction 
of a heliometer. An ordinary telescope lens is 
sawed in half by means of a thin round metal 
disk revolved rapidly by machinery, and fed con- 
tinually with emery and water at its edge. The 
cutting effect of emery is sufficient to make such 
a disk enter glass much as an ordinary saw pene- 
trates wood. The two " semi-lenses," as they 
are called, are then mounted separately in metal 
holders. These are attached to one end of the 
hehometer, called the " head," In such a way that 
the two semi-lenses can slide side by side upon 
metal guides. This head is then fastened to one 
end of a telescope tube mounted in the usual 



way. The " head " end of the instrument is 
turned toward the sky in observing, and at the 
eye-end is placed the usual little magnifier we 
have already described. 

The observer at the eye-end has control of 
certain rods by means of which he can push the 
semi-lenses on their slides in the head at the 
other end of the tube. Now, if he moves the 
semi-lenses so as to bring them side by side ex- 
actly, the whole arrangement will act like an or- 
dinary telescope. For the semi-lenses will then 
fit together just as if the original glass had never 
been cut. But if the half-lenses are separated a 
little on their slides, each will act by itself. Be- 
ing slightly separated, each will cover a different 
part of the sky. The whole affair acts as if the 
observer at the eye-end were looking through 
two telescopes at once. For each semi-lens acts 
independently, just as if it were a complete glass 
of only half the size. 

Now, suppose there were a couple of stars in 
the sky, one in the part covered by the first 
semi-lens, and one in the part covered by the 
second. The observer would, of course, see 



both stars at once upon looking into the little 
magnifier at the eye-end of the heliometer. 

We must remember that these stars will 
appear in the field of view simply as two tiny 
points of light. The astronomer, as we have 
said, is provided with a simple system of long 
rods, by means of which he can manipulate the 
semi-lenses while the observation is being made. 
If he slides them very slowly one way or the 
other, the two star-points in the field of view will 
be seen to approach each other. In this way 
they can at last be brought so near together that 
they will form but a single dot of light. Obser- 
vation with the heliometer consists in thus bring- 
ing two star-images together, until at last they 
are superimposed one upon the other, and we see 
one image only. Means are provided by which 
it is then possible to measure the amount of 
separation of the two half-lenses. Evidently the 
farther asunder on the sky are the two stars 
under observation, the greater will be the separa- 
tion of the semi-lenses necessary to make a single 
image of their light. Thus, measurement of the 
lenses' separation becomes a means of determin- 



ing the separation of the stars themselves upon 
the sky. 

The two slides in the heliometer head are sup- 
plied with a pair of very delicate measures or 
" scales " usually made of silver. These can be 
examined from the eye-end of the instrument by 
looking through a long microscope provided for 
this special purpose. Thus an extremely precise 
value is obtained both of the separation of the 
sliders and of the distance on the sky between 
the stars under examination. Measures made in 
this way with the heliometer are counted the 
most precise of astronomical observations. 

Having thus described briefly the kind of ob- 
servations obtained with the heliometer, we shall 
now touch upon their further utilization. We 
shall take as an example but one of their many 
uses — that one which astronomers consider the 
most important — the measurement of stellar dis- 
tances. (See also p. 94.) 

Even the nearest fixed star is almost incon- 
ceivably remote from us. And astronomers 
are imprisoned on this little earth ; we cannot 
bridge the profound distance separating us from 



the stars, so as to use direct measurement with 
tape-line or surveyor's chain. We are forced to 
have recourse to some indirect method. Suppose 
a certain star is suspected, on account of its bright- 
ness, or for some other reason, of being near us 
in space, and so giving a favorable opportunity 
for a determination of distance. A couple of 
very faint stars are selected close by. These, on 
account of their faintness, the astronomer may 
regard as quite immeasurably far away. He then 
determines with his heliometer the exact position 
on the sky of the bright star with respect to the 
pair of faint ones. Half a year is then allowed 
to pass. During that time the earth has been 
swinging along in its annual path or orbit around 
the sun. Half a year will have sufficed to carry 
the observer on the earth to the other side of 
that path, and he is then 185,000,000 miles away 
from his position at the first observation. 

Another determination is made of the bright 
star's position as referred to the two faint ones. 
Now, if all these stars were equally distant, their 
relative positions at the second observation would 
be just the same as at the former one. But if, as 



is very probable, the bright star is very much 
nearer us than are the two faint ones, we shall 
obtain a different position from our second ob- 
servation. For the change of 185,000,000 miles 
in the observer's location will, of course, affect 
the direction in which we see the near star, while 
it will leave the distant ones practically unchanged. 
Without entering into technical details, we may 
say that from a large number of observations of 
this kind, we can obtain the distance of the bright 
star by a process of calculation. The only es- 
sential is to have an instrument that can make 
the actual observations of position accurately 
enough ; and in this respect the heliometer is still 



Scarcely anyone can have watched the sky 
without noticing how different is the behavior of 
our moon from that of any other object we can 
see. Of course, it has this in common with the 
sun and stars and planets, that it rises in the 
eastern horizon, slowly climbs the dome of the 
sky, and again goes down and sets in the west. 
This motion of the heavenly bodies is known to 
be an apparent one merely, and caused by the 
turning of our own earth upon its axis. A man 
standing upon the earth's surface can look up 
and see above him one-half the great celestial 
vault, gemmed with twinkling stars, and bearing, 
perhaps, within its ample curve one or two se- 
renely shining planets and the lustrous moon. 
But at any given moment the observer can see 
nothing of the other half of the heavenly sphere. 
It is beneath his feet, and concealed by the solid 
bulk of the earth. 



The earth, however, is turning on an axis, 
carrying the observer with it. And so it is con- 
tinually presenting him to a new part of the sky. 
At any moment he sees but a single half-sphere ; 
yet the very next instant it is no longer the same ; 
a small portion has passed out of sight on one 
side by going down behind the turning earth, 
whUe a corresponding new section has come into 
view on the opposite side. It is this coming into 
view that we call the rising of a star ; and the 
corresponding disappearance on the other side is 
called setting. Thus rising and setting are, of 
course, due entirely to a turning of the earth, and 
not at all to actual motions of the stars ; and for 
this reason, all objects in the sky, without ex- 
ception, must rise and set again. But the moon 
really has a motion of its own in addition to this 
apparent one caused by the earth's rotation. 

Somewhere in the dawn of time early watchers 
of the stars thought out those fancied constella- 
tions that survive even down to our own day. 
They placed the mighty lion, king of beasts, 
upon the face of night, and the great hunter, too, 

armed with club and dagger, to pursue him. 



Among these constellations the moon threads her 
destined way, night after night, so rapidly that 
the unaided eye can see that she is moving. It 
needs but little power of fancy's magic to recall 
from the dim past a picture of some aged astron- 
omer graving upon his tablets the Records of 
the Moon. " To-night she is near the bright 
star in the eye of the Bull." And again : " To- 
night she rides full, and near to the heart of the 

And why does the moon ride thus through the 
stars of night ? Modern science has succeeded 
in disentangling the intricacies of her motion, 
until to-day only one or two of the very minutest 
details of that motion remain unexplained. But 
it has been a hard problem. Someone has well 
said that lunar theory should be likened to a 
lofty cliff upon whose side the intellectual giants 
among men can mark off their mental stature, 
but whose height no one of them may ever hope 
to scale. 

But for our present purpose it is unnecessary 
to pursue the subject of lunar motion into its 
abstruser details. To understand why the moon 



moves rapidly among the stars, it is sufficient to 
remember that she is whirling quickly round the 
earth, so as to complete her circuit in a little less 
than a month. We see her at all times projected 
upon the distant background of the sky on which 
are set the stellar points of light, as though in- 
tended for beacons to mark the course pursued 
by moon and planets. The stars themselves have 
no such motions as the moon ; situated at a dis- 
tance almost inconceivably great, they may, in- 
deed, be travellers through empty space, yet their 
journeys shrink into insignificance as seen from 
distant earth. It requires the most delicate in- 
struments of the astronomer to so magnify the 
tiny displacements of the stars on the celestial 
vault that they may be measured by human eyes. 
Let us again recur to our supposed observer 
watching the moon night after night, so as to 
record the stars closely approached by her. Why 
should he not some time or other be surprised 
by an approach so close as to amount apparent- 
ly to actual contact ? The moon covers quite 
a large surface on the sky, and when we remem- 
ber the nearly countless numbers of the stars, it 



would, indeed, be strange if there were not some 
behind the moon as well as all around her. 

A moment's consideration shows that this must 
often be the case ; and in fact, if the moon's ad- 
vancing edge be scrutinized carefully through a 
telescope, small stars can be seen frequently to 
disappear behind it. This is a most interesting 
observation. At first we see the moon and star 
near each other in the telescope's field of view. 
But the distance between them lessens percepti- 
bly, even quickly, until at last, with a startling 
suddenness, the star goes out of sight behind the 
moon. After a time, ranging from a few mo- 
ments to, perhaps, more than an hour, the moon 
will pass, and we can see the star suddenly reap- 
pear from behind the other edge. 

These interesting observations, while not at all 
uncommon, can be made only very rarely by 
naked-eye astronomers. The reason is simple. 
The moon's light is so brilliant that it fairly over- 
comes the stars whenever they are at all near, ex- 
cept in the case of very bright ones. Small 
stars that can be followed quite easily up to the 
moon's edge in a good telescope, disappear from 



a naked-eye view while the moon is still a long 
distance away. But the number of very bright 
stars is comparatively small, so that it is quite un- 
usual to find anyone not a professional astrono- 
mer who has actually seen one of these so-called 
" occultations." Moreover, most people are not 
informed in advance of the occurrence of an op- 
portunity to make such observations, although 
they can be predicted quite easily by the aid of 
astronomical calculations. Sometimes we have 
occultations of planets, and these are the most in- 
teresting of all. When the moon passes between 
us and one of the larger planets, it is worth 
while to observe the phenomenon even without a 

Up to this point we have considered occulta- 
tions chiefly as being of interest to the naked-eye 
astronomer. Yet occultations have a real scien- 
tific value. It is by their means that we can, 
perhaps, best measure the moon's size. By not- 
ing with a telescope the time of disappearance 
and reappearance of known stars, astronomers 
can bring the direct measurement of the moon's 
diameter within the range of their numerical cal- 



culations. Sometimes the moon passes over a 
condensed cluster of stars like the Pleiades. 
The results obtainable on these occasions are 
valuable in a very high degree, and contribute 
largely to making precise our knowledge of the 
lunar diameter. 

There is another thing of scientific interest 
about occultations, though it has lost some of 
its importance in recent years. When such an 
event has been observed, the agreement of the 
predicted time with that actually recorded by the 
astronomer offers a most searching test of the 
correctness of our theory of lunar motion. We 
have already called attention to the great inher- 
ent difficulty of this theory. It is easy to see 
that by noting the exact instant of disappearance 
of a star at a place on the earth the latitude and 
longitude of which are known, we can both 
check our calculations and gather material for 
improving our theory. The same principle can 
be used also in the converse direction. Within 
the limits of precision imposed by the state of 
our knowledge of lunar theory, we can utilize 

occultations to help determine the position on the 



earth of places whose longitude is unknown. It 
is a very interesting bit of history that the first 
determination of the longitude of Washington 
was made by means of occultations, and that this 
early determination led to the founding of the 
United States Naval Observatory. 

On March 28, 18 10, Mr. Pitkin, of Connec- 
ticut, reported to the House of Representatives 
on " laying a foundation for the establishment of 
a first meridian for the United States, by which 
a further dependence on Great Britain or any 
other foreign nation for such meridian may be 
entirely removed." This report was the result 
of a memorial presented by one William Lam- 
bert, who had deduced the longitude of the Cap- 
itol from an occultation observed October 20, 
1804. Various proceedings were had in Con- 
gress and in committee, until at last, in 1821, 
Lambert was appointed " to make astronomical 
observations by lunar occultations of fixed stars, 
solar eclipses, or any approved method adapted 
to ascertain the longitude of the Capitol from 
Greenwich." Lambert's reports were made in 

1822 and 1823, but ten years passed before the 



establishment of a formal Naval Observatory un- 
der Goldsborough, Wilkes, and Gilliss. But to 
Lambert belongs the honor of having marked 
out the first fundamental official meridian of 
longitude in the United States. 



There are many interesting practical things 
about an astronomical observatory with which 
the public seldom has an opportunity to become 
acquainted. Among these, perhaps, the details 
connected with setting up a great telescope take 
first rank. The writer happened to be present 
at the Cape of Good Hope Observatory when 
the photographic equatorial telescope was being 
mounted, and the operation of putting it in posi- 
tion may be taken as typical of similar processes 
elsewhere. (See also p. 86.) 

In the first place, it is necessary to explain 

what is meant by an " equatorial " telescope. 

One of the chief difficulties in making ordinary 

observations arises from the rising and setting of 

the stars. They are all apparently moving across 

the face of the sky, usually climbing up from the 

eastern horizon, only to go down again and set 

in the west. If, therefore, we wish to scrutinize 


Forty-Inch Telescope, Yerkes Observatory, 
University of Chicago. 


any given object for a considerable time, we must 
move the telescope continuously so as to keep 
pace with the motion of the heavens. For this 
purpose, the tube must be attached to axles, so 
that it can be turned easily in any direction. 
The equatorial mounting is a device that permits 
the telescope to be thus aimed at any part of the 
sky, and at the same time facilitates greatly the 
operation of keeping it pointed correctly after a 
star has once been brought into the field of view. 

To understand the equatorial mounting it is 
necessary to remember that the rising and setting 
motions of the heavenly bodies are apparent ones 
only, and due in reality to the turning of the 
earth on its own axis. As the earth goes around, 
it carries observer, telescope, and observatory past 
the stars fixed upon the distant sky. Conse- 
quently, to keep a telescope pointed continuously 
at a given star, it is merely necessary to rotate it 
steadily backward upon a suitable axis just fast 
enough to neutralize exactly the turning of our 

By a suitable axis for this purpose we mean 

one so mounted as to be exactly parallel to the 



earth's own axis of rotation. A little reflection 
shows how simply such an arrangement will 
work. All the heavenly bodies may be re- 
garded, for practical purposes, as excessively re- 
mote in comparison with the dimensions of our 
earth. The entire planet shrinks into absolute 
insignificance when compared with the distances 
of the nearest objects brought under observation 
by astronomers. It follows that if we have our 
telescope attached to such a rotation-axis as we 
have described, it will be just the same for pur- 
poses of observation as though the telescope's 
axis were not only parallel to the earth's axis, but 
actually coincident with it. The two axes may 
be separated by a distance equal to that between 
the earth's surface and its centre ; but, as we 
have said, this distance is insignificant so far as 
our present object is concerned. 

There is another way to arrive at the same 
result. We know that the stars in rising and 
setting all seem to revolve about the pole star, 
which itself seems to remain immovable. Con- 
sequently, if we mount our telescope so that it 

can turn about an axis pointing at the pole, we 



shall be able to neutralize the rotation of the stars 
by simply turning the telescope about the axis at 
the proper speed and in the right direction. As- 
tronomical considerations teach us that an axis 
thus pointing at the pole will be parallel to the 
earth's own axis. Thus we arrive at the same 
fundamental principle for mounting an astronom- 
ical telescope from whichever point of view we 
cojisider the subject. 

Every large telescope is provided with such an 
axis of rotation ; and for the reason stated it is 
called the " polar axis." The telescope Itself is 
then called an " equatorial." The advantage of 
this method of mounting is very evident. Since 
we can follow the stars' motions by turning the 
telescope about one axis only, it becomes a very 
simple matter to accomplish this turning auto- 
matically by means of clock-work. 

The " following " of a star being thus provided 
for by the device of a polar axis, it is, of course, 
also necessary to supply some other motion so as 
to enable us to aim the tube at any point in the 
heavens. For it is obvious that if it were rigidly 
attached to the polar axis, we could, indeed, follow 



any star that happened to be in the field of view, 
but we could not change this field of view at will 
so as to observe other stars or planets. To ac- 
complish this, the telescope is attached to the 
polar axis by means of a pivot. By turning the 
telescope around its polar axis, and also on this 
pivot, we can find any object in the heavens ; and 
once found, we can leave to the polar axis and its 
automatic clock-work the task of keeping that 
object before the observer's eye. 

In setting up the Cape of Good Hope instru- 
ment the astronomers were obliged to do a large 
part of the work of adjustment personally. Far 
away from European instrument-makers, the parts 
of the mounting and telescope had to be " assem- 
bled," or put together, by the astronomers of the 
Cape Observatory. A heavy pier of brick and 
masonry had been prepared in advance. Upon 
this was placed a massive iron base, intended to 
support the superstructure of polar axis and tele- 
scope. This base rested on three points, one of 
which could be screwed in and out, so as to tilt 
the whole affair a little forward or backward. 
By means of this screw we effected the final ad- 



justment of the polar axis to exact parallelism 
with that of the earth. Other screws were pro- 
vided with which the base could be twisted a little 
horizontally either to the right or left. Once set 
up in a position almost correct, it was easy to per- 
fect the adjustment by the aid of these screws. 

Afterward the tube and lenses were put in 
place, and the clock properly attached inside the 
big cast-iron base. This clock-work looked more 
like a piece of heavy machinery than a delicate 
clock mechanism. But it had heavy work to do, 
carrying the massive telescope with its weighty 
lenses, and needed to be correspondingly strong. 
It had a driving-weight of about 2,000 pounds, 
and was so powerful that turning the telescope 
affected it no more than the hour-hand of an ordi- 
nary clock affects the mechanism within its case. 

The final test of the whole adjustment con- 
sisted in noting whether stars once brought into 
the telescopic field of view could be maintained 
there automatically by means of the clock. This 
object having been attained successfully, the in- 
strument stood ready to be used in the routine 
business of the observatory. 



Before leaving the subject of telescope-mount- 
ings, we must mention the giant instrument set 
up at the Paris Exposition of 1900. The project 
of having a Grande Lunette had been hailed by 
newspapers throughout the world and by the 
general public in their customary excitable way. 
It was tremendously over-advertised ; exagger- 
ated notions of the instrument's powers were 
spread abroad and eagerly credited ; the moon 
was to be dragged down, as it were, from its 
customary place in the sky, so near that we 
should be able almost to touch its surface. As 
to the planets — free license was given to the jour- 
nalistic imagination, and there was no effective 
limitation to the magnificence of astronomical 
discovery practically within our grasp, beyond 
the necessity for printed space demanded by sun- 
dry wars, pestilences, and other mundane trifles. 

Now, the present writer is very far from advo- 
cating a lessening of the attention devoted to as- 
tronomy. Rather would he magnify his office 
than diminish it. But let journalistic astronomy 
be as good an imitation of sober scientific truth 

as can be procured at space rates ; let editors en- 



courage the public to study those things in the 
science that are ennobling and cultivating to the 
mind ; let there be an end to the frenzied effort 
to fabricate a highly colored account of alleged 
discoveries of yesterday, capable of masquerad- 
ing to-day under heavy head-lines as News. 

The manner in which the big telescope came 
to be built is not without interest, and shows 
that enterprise is far from dead, even in the old 
countries, A stock company was organized — 
we should call it a corporation — under the 
name Societe de FOptique. It would appear that 
shares were regularly put on the market, and 
that a prospectus, more or less alluring, was 
widely distributed. We may say at once that the 
investing public did not respond with obtrusive 
alacrity ; but at all events, the promoters' efforts 
received sufficient encouragement to enable them 
to begin active work. From the very first a 
vigorous attempt was made to utilize both the 
resources of genuine science and the devices of 
quasi-charlatanry. It was announced that the 
public were to be admitted to look through the 
big glass (apparently at so much an eye), and 



many, doubtless, expected that the man in the 
street would be able to make personal acquaint- 
ance with the man in the moon. A telescopic 
image of the sun was to be projected on a big 
screen, and exhibited to a concourse of specta- 
tors assembled in rising tiers of seats within a 
great amphitheatre. And when clouds or other 
circumstances should prevent observing the plan- 
ets or scrutinizing the sun, a powerful stereopti- 
con was to be used. Artificial pictures of the 
wonders of heaven were to be projected on the 
screen, and the public would never be disap- 
pointed. It was arranged that skilled talkers 
should be present to explain all marvels : and, in 
short, financial profit was to be combined with 
machinery for advancing scientific discovery. 
Astronomers the world over were "circularized," 
asked to become shareholders, and, in default of 
that^ to send lantern-slides or photographs of 
remarkable celestial objects for exhibition in the 
magic-lantern part of the show. 

The project thus brought to the attention of 
scientific men three years ago did not have an 

attractive a r. It savored too much of charlatan- 



ism. But it soon appeared that effective gov- 
ernment sanction had been given to the enter- 
prise ; and, above all, that men of reputation 
were allowing the use of their names in connec- 
tion with the affair. More important still, we 
learned that the actual construction had been 
undertaken by Gautier, of Paris, that finances 
were favorable, and that real work on parts of 
the instrument was to commence without delay. 

Gautier is a first-class instrument-builder; he 
has established his reputation by constructing suc- 
cessfully several telescopes of very large size, in- 
cluding the equatorial coude of the Paris Observa- 
tory, a unique instrument of especial complexity. 
The present writer believes that, if sufficient time 
and money were available, the Grande Lunette 
would stand a reasonable chance of success in 
the hands of such a man. And by a reasonable 
chance, we mean that there is a large enough 
probability of genuine scientific discovery to 
justify the necessary financial outlay. But the 
project should be divorced from its " popular " 
features, and every kind of advertising and char- 
latanism excluded with rigor. 



As planned originally, and actually constructed, 
the Grande Lunette presents interesting peculiari- 
ties, distinguishing it from other telescopes. Pre- 
vious instruments have been built on the prin- 
ciple of universal mobility. It is possible to 
move them in all directions, and thus bring any 
desired star under observation, irrespective of its 
position in the sky. But this general mobility 
offers great difficulties in the case of large and 
ponderous telescopes. Delicacy of adjustment is 
almost destroyed when the object to be adjusted 
weighs several tons. And the excessive weight 
of telescopes is not due to unavoidably heavy 
lenses alone. It is essential that the tube be 
long; and great length involves appreciable 
thickness of material, if stiffness and solidity are 
to remain unsacrificed. Length in the tube is 
necessitated by certain peculiar optical defects of 
all lenses, into the nature of which we shall not 
enter at present. The consequences of these 
defects can be rendered harmless only if the in- 
strument is so arranged that the observer's eye is 
far from the other end of the tube. The length 
of a good telescope should be at least twelve 



times the diameter of its large lens. If the rela- 
tive length can be still further increased, so 
much the better ; for then the optical defects 
can be further reduced. <s</ *^'-t. tu^ ^ Jju^a,jJ. ' 

In the case of the Paris instrument a radical 
departure consists in making the tube of unprec- 
edented length, 197 feet, with a lens diameter of 
49% inches. This great length, while favorable 
optically, precludes the possibility of making the 
instrument movable in the usual sense. In fact, 
the entire tube is attached to a fixed horizontal 
base, and no attempt is made to change its posi- 
tion. Outside the big lens, and disconnected 
altogether from the telescope proper, is mounted 
a smooth mirror, so arranged that it can be 
turned in any direction, and thus various parts of 
the sky examined by reflection in the telescope. 

While this method unquestionably has the ad- 
vantage of leaving the optician quite free as to 
how long he will make his tube, it suffers from 
the compensating objection that a new optical 
surface is introduced into the combination, viz., 
the mirror. Any slight unavoidable imperfec- 
tion in the polishing of its surface will infallibly 



be reproduced on a magnified scale in the image 
of a distant star brought before the observer's 

But it is not yet possible to pronounce defi- 
nitely upon the merit of this form of instrument, 
since, as we have said, the maker has not been 
given time enough to try the idea to the com- 
plete satisfaction of scientific men. In the early 
part of August, 1900, when the informant of the 
prtsent writer left Paris, after serving as a mem- 
ber of the international jury for judging instru- 
ments of precision at the Exposition, the condi- 
tion of the Grande Lunette was as follows : Two 
sets of lenses had been contemplated, one intend- 
ed for celestial photography, and the other to be 
used for ordinary visual observation. Only the 
photographic lenses had been completed, how- 
ever, and for this reason the public could not be 
permitted to look through the instrument. The 
photographic lenses were in place in the tube, but 
at that time their condition was such that, though 
some photographs had been obtained, it was not 
thought advisable to submit them to the jury. 

Consequently, the Lunette did not receive a 



prize. Since that time various newspapers have 
reported wonderful results from the telescope ; 
but, disregarding the fusillade from the sensa- 
tional press, we may sum up the present state of 
affairs very briefly. Gautier is still experiment- 
ing ; and, given sufficient time and money, he 
may succeed in producing what astronomers 
hope for — an instrument capable of advancing 
our knowledge, even if that advance be only a 
small one. 



The pole of the frozen North is not the only 
pole sought with determined effort by more than 
one generation of scientific men. Up in the sky 
astronomers have another pole which they are 
following up just as vigorously as ever Arctic ex- 
plorer struggled toward the difficult goal of his ter- 
restrial journeying. The celestial pole is, indeed, 
a fundamentally important thing in astronomical 
science, and the determination of its exact position 
upon the sky has always engaged the closest 
attention of astronomers. Quite recently new 
methods of research have been brought to bear, 
promising a degree of success not hitherto at- 
tained in the astronomers' pursuit of their pole. 

In the first place, we must explain what is 
meant by the celestial pole. We have already 
mentioned the poles of the earth (p. 136). Our 
planet turns once daily upon an axis passing 
through its centre, and it is this rotation that 



causes all the so-called diurnal phenomena of the 
heavens. Rising and setting of sun, moon, and 
stars are simply results of this turning of the 
earth. Heavenly bodies do not really rise ; it is 
merely the man on the earth who is turned round 
on an axis until he is brought into a position from 
which he can see them. The terrestrial poles are 
those two points on the earth's surface where it 
is pierced by the rotation axis of the planet. 
Now we can, if we choose, imagine this axis 
lengthened out indefinitely, further and further, 
until at last it reaches the great round vault of 
the sky. Here it will again pierce out two polar 
points ; and these are the celestial poles. 

The whole thing is thus quite easy to un- 
derstand. On the sky the poles are marked by 
the prolongation of the earth's axis, just as on 
the earth the poles are marked by the axis itself. 
And this explains at once why the stars seem 
nightly to revolve about the pole. If the ob- 
server is being turned round the earth's axis, of 
course it will appear to him as if the stars were 
rotating around the same axis in the opposite 
direction, just as houses and fields seem to fly 



past a person sitting in a railway train, unless he 
stops to remember that it is really himself who is 
in motion, and not the trees and houses. 

The existence of such a centre of daily motions 
among the stars once recognized, it becomes of 
interest to ascertain whether the centre itself al- 
ways retains precisely the same position in the 
sky. It was discovered as early as the time of 
Hipparchus (p. 39) that such is not the case, 
and that the celestial pole is subject to a slow 
motion among the stars on the sky. If a given 
star were to-day situated exactly at the pole, it 
would no longer be there after the lapse of a 
year's time ; for the pole would have moved away 
from it. 

This motion of the pole is called precession. 
It means that certain forces are continually at 
work, compelling the earth's axis to change its 
position, so that the prolongation of that axis 
must pierce the sky at a point which moves as 
time goes on. These forces are produced by the 
gravitational attractions of the sun, moon, and 
planets upon the matter composing our earth. 

If the earth were perfectly spherical in shape, 



the attractions of the other heavenly bodies 
would not affect the direction of the earth's rota- 
tion-axis in the least. But the earth is not quite 
globular in form ; it is flattened a little at the 
poles and bulges out somewhat at the equator. 
(See p. 135.) 

This protuberant matter near the equator gives 
the other bodies in the solar system an oppor- 
tunity to disturb the earth's rotation. The gen- 
eral effect of all these attractions is to make the 
celestial pole move upon the sky in a circle hav- 
ing a radius of about 23 ^ degrees ; and it re- 
quires 25,800 years to complete a circuit of this 
precessional cycle. One of the most striking con- 
sequences of this motion will be the change of the 
polar star. Just at present the bright star Polaris 
in the constellation of the Little Bear is very 
close to the pole. But after the lapse of sufHcient 
ages the first-magnitude star Vega of the constella- 
tion Lyra will in its turn become Guardian of the 

It must not be supposed, however, that the 
motion of the pole proceeds quite uniformly, and 
in an exact circle ; the varying positions of the 



heavenly bodies whose attractions cause the phe- 
nomena in question are such as to produce ap- 
preciable divergencies from exact circular motion. 
Sometimes the pole deviates a little to one side 
of the precessional circle, and sometimes it de- 
viates on the other side. The final result is a 
sort of wavy line, half on one side and half on 
the other of an average circular curve. It takes 
only nineteen years to complete one of these little 
waves of polar motion, so that in the whole pre- 
cessional cycle of 25,800 years there are about 
1 ,400 indentations. This disturbance of the polar 
motion is called by astronomers nutation. 

The first step in a study of polar motion is to 
devise a method of finding just where the pole is 
on any given date. If the astronomer can ascer- 
tain by observational processes just where the 
pole is among the stars at any moment, and can 
repeat his observations year after year and genera- 
tion after generation, he will possess in time a 
complete chart of a small portion at least of the 
celestial pole's vast orbit. From this he can ob- 
tain necessary data for a study of the mathematical 

theory of attractions, and thus, perhaps, arrive at 



an explanation of the fundamental laws governing 
the universe in which we live. 

The instrument which has been used most ex- 
tensively for the study of these problems is the 
transit (p. ii8)orthe "meridian circle." This 
latter consists of a telescope firmly attached to a 
metallic axis about which it can turn. The axis 
itself rests on massive stone supports, and is so 
placed that it points as nearly as possible in an 
east-and-west direction. Consequently, when the 
telescope is turned about its axis, it will trace out 
on the sky a great circle (the meridian) which 
passes through the north and south points of the 
horizon and the point directly overhead. The 
instrument has also a metallic circle very firmly 
fastened to the telescope and its axis. Let into 
the surface of this circle is a silver disk upon 
which are engraved a series of lines or gradua- 
tions by means of which it is impossible to meas- 
ure angles. 

Observers with the meridian circle begin by 
noting the exact instant when any given star 
passes the centre of the field of view of the tele- 
scope. This centre is marked with a cross made 



by fastening into the focus some pieces of ordi- 
nary spider's web, which give a well-marked, deli- 
cate set of lines, even under the magnifying power 
of the telescope's eye-piece. In addition to thus 
noting the time when the star crosses the field 
of the telescope, the astronomer can measure by 
means of the circle, how high up it was in the sky 
at the instant when it was thus observed. 

If the telescope of the meridian circle be turned 
toward the north, and we observe stars close to 
the pole, it is possible to make two different ob- 
servations of the same star. For the close polar 
stars revolve in such small circles around the pole 
of the heavens that we can observe them when 
they are on the meridian either above the pole or 
below it. Double observations of this class en- 
able us to obtain the elevation of the pole above 
the horizon, and to fix its position with respect 
to the stars. 

Now, there is one very serious objection to 
this method. In order to secure the two neces- 
sary observations of the same star, it is essential 
to be stationed at the instrument at two moments 
of time separated by exactly twelve hours ; and if 



one of the observations occurs in the night, the 
other corresponding observation will occur in 

It is a fact not generally known that the 
brighter stars can be seen with a telescope, even 
when the sun is quite high above the horizon. 
Unfortunately, however, there is only one star 
close to the pole which is bright enough to be 
thus observed in daylight — the polar star already 
mentioned under the name Polaris. The fact 
that we are thus limited to observations of a single 
star has made it difficult even for generations of 
astronomers to accumulate with the meridian cir- 
cle a very large quantity of observational material 
suitable for the solution of our problem. 

The new method of observation to which we 
have referred above consists in an application of 
photography to the polar problem. If we aim 
at the pole a powerful photographic telescope, 
and expose a photographic plate throughout the 
entire night, we shall find that all stars coming 
within the range of the plate will mark out little 
circles or " trails " upon the developed negative. 
It is evident that as the stars revolve about the 



pole on the sky, tracing out their daily circular 
orbits, these same little circles must be repro- 
duced faithfully upon the photographic plate. 
The only condition is that the stars shall be 
bright enough to make their light affect the sen- 
sitive gelatine surface. 

But even if observations of this kind are con- 
tinued throughout all the hours of darkness, we 
do not obtain complete circles, but only those 
portions of circles traced out on the sky between 
sunset and sunrise. If the night is twelve hours 
in length, we get half-circles on the plate ; if it is 
eighteen hours long, we get circles that lack only 
one-quarter of being complete. In other words, 
we get a series of circular arcs, one corresponding 
to each close polar star. There are no fewer than 
sixteen stars near enough to the pole to come 
within the range of a photographic plate, and 
bright enough to cause measurable impressions 
upon the sensitive surface. The fact that the 
circular arcs are not complete circles does not in 
the least prevent our using them for ascertaining 
the position of their common centre ; and that 
centre is the pole. Moreover, as the arcs are 



distributed at all sorts of distances from the pole 
and in all directions, corresponding to the acci- 
dental positions of the stars on the sky, we have 
a state of affairs extremely favorable to the ac- 
curate determination of the pole's place among 
the stars by means of microscopic measurements 
of the plate. 

It will be perceived that this method is ex- 
tremely simple, and, therefore, likely to be suc- 
cessful ; though its simplicity is slightly impaired 
by the phenomenon known to astronomers as 
" atmospheric refraction." The rays of light 
coming down to our telescopes from a distant 
star must pass through the earth's atmosphere 
before they reach us ; and in passing thus from 
the nothingness of outer space into the denser 
material of the air, they are bent out of their 
straight course. The phenomenon is analogous 
to what we see when we push a stick down 
through the surface of still water ; we notice that 
the stick appears to be bent at the point where it 
pierces the surface of the water ; and in just the 
same way the rays of light are bent when they 
pierce into the air. Fortunately, the mathemati- 



cal theory of this atmospheric bending of light is 
well understood, so that it is possible to remove 
the effects of refraction from our results by a 
process of calculation. In other words, we can 
transform our photographic measures into what 
they would have been if no such thing as at- 
mospheric refraction existed. This having been 
done, all the arcs on the plate should be exactly 
circular, and their common centre should be the 
position of the pole among the stars on the night 
when the photograph was made. 

It is possible to facilitate the removal of re- 
fraction effects very much by placing our photo- 
graphic telescope at some point on the earth situ- 
ated in a very high latitude. The elevation of 
the pole above the horizon is greatest in high 
latitudes. Indeed, if Arctic voyagers could ever 
reach the pole of the earth they would see the 
pole of the heavens directly overhead. Now, 
the higher up the pole i^ in the sky, the less will 
be the effects of atmospheric refraction ; for the 
rays of light will then strike the atmosphere in a 
direction nearly perpendicular to its surface, which 

is favorable to diminishing the amount of bending. 



There is also another very important ad- 
vantage in placing the telescope in a high lati- 
tude ; in the middle of winter the nights are very 
long there ; if we could get within the Arctic 
Circle itself, there would be nights when the 
hours of darkness would number twenty-four, 
and we could substitute complete circles for our 
broken arcs. This would, indeed, be most fa- 
vorable from the astronomical point of view; but 
the essential condition of convenience for the ob- 
server renders an expedition to the frozen Arctic 
regions unadvisable. 

But it is at least possible to place the tele- 
scope as far north as is consistent with retaining 
it within the sphere of civilized influences. We 
can put it in that one of existing observatories on 
the earth which has the highest latitude ; and 
this is the observatory of Helsingfors, in Fin- 
land, which belongs to a great university, is 
manned by competent astronomers, and has a 
latitude greater than 60 degrees. 

Dr. Anders Donner, Director of the Helsing- 
fors Observatory, has at its disposal a fine photo- 
graphic telescope, and with this some prelimi- 



nary experimental "trail" photographs were made 
in 1895. These photographs were sent to Co- 
lumbia University, New York, and were there 
measured under the writer's direction. Calcula- 
tions based on these measures indicate that the 
method is promising in a very high degree ; and 
it was, therefore, decided to construct a special 
photographic telescope better adapted to the par- 
ticular needs of the problem in hand. 

The desirability of a new telescope arises from 
the fact that we wish the instrument to remain ab- 
solutely unmoved during all the successive hours 
of the photographic exposure. It is clear that 
if the telescope moves while the stars are tracing 
out their little trails on the plate, the circularity 
of the curves will be disturbed. Now, ordinary 
astronomical telescopes are always mounted upon 
very stable foundations, well adapted to making 
the telescope stand still ; but the polar telescope 
which we wish to use in a research fundamental 
to the entire science of astronomy ought to pos- 
sess immobility and stability of an order higher 
than that required for ordinary astronomical pur- 



It is a remarkable peculiarity of the instru- 
ment needed for the new trail photographs that 
it is never moved at all. Once pointed at the 
pole, it is ready for all the observations of suc- 
cessive generations of astronomers. It should 
have no machinery, no pivots, axes, circles, 
clocks, or other paraphernalia of the usual equa- 
torial telescope. All we want is a very heavy 
stone pier, with a telescope tube firmly fastened 
to it throughout its entire length. The top of 
the pier having been cut to the proper angle of 
the pole's elevation, and the telescope cemented 
down, everything is complete from the instru- 
mental side ; and just such an instrument as 
this is now ready for use at Helsingfors. 

The late Miss Catharine Wolfe Bruce, of 
New York, was much interested in the writer's 
proposed polar investigations, and in October, 
1898, she contributed funds for the construction 
of the new telescope, and the Russian authori- 
ties have generously undertaken the expense of a 
building to hold the instrument and the granite 
foundation upon which it rests. Photographs 

are now being secured with the new instrument, 



and they will be sent to Columbia University, 
New York, for measurement and discussion. It 
is hoped that they will carry out the promise of 
the preliminary photographs made in 1895 ^^^^ 
a less suitable telescope of the ordinary form. 



The public attitude toward matters scientific 
is one of the mysteries of our time. It can be 
described best by the single word. Credulity ; 
simple, absolute credulity. Perfect confidence is 
the most remarkable characteristic of this unbe- 
lieving age. No charlatan, necromancer, or as- 
trologer of three centuries ago commanded more 
respectful attention than does his successor of 

Any person can be a scientific authority; he 
has but to call himself by that title, and every- 
one will give him respectful attention. Numer- 
ous instances can be adduced from the experience 
of very recent years to show how true are these 
remarks. We have had the Keeley motor and 
the liquid-air power schemes for making some- 
thing out of nothing. Extracting gold from sea- 
water has been duly heralded on scientific author- 
ity as an easy source of fabulous wealth for the 



million. Hard-headed business men not only 
believe in such things, but actually invest in them 
their most valued possession, capital. Venders 
of nostrums and proprietary medicines acquire 
wealth as if by magic, though it needs but a mo- 
ment's reflection to realize that these persons can- 
not possibly be in possession of any drugs, or 
secret methods of compounding drugs, that are 
unknown to scientific chemists. 

If the world, then, will persistently intrust its 
health and wealth into the safe-keeping of charla- 
tans, what can we expect when things supposedly 
of far less value are at stake ? The famous Moon 
Hoax, as we now call it, is truly a classic piece of 
lying. Though it dates from as long ago as 
1835, ^' ^*® never had an equal as a piece of 
" modern " journalism. Nothing could be more 
useful than to recall it to public attention at least 
once every decade; for it teaches an important 
lesson that needs to be iterated again and again. 

On November 13, 1833, Sir John Herschel 

embarked on the Mountstuart Elphinstone, bound 

for the Cape of Good Hope. He took with 

him a collection of astronomical instruments, 



with which he intended to study the heavens 
of the southern hemisphere, and thus extend 
his father's great work to the south polar stars. 
An earnest student of astronomy, he asked no 
better than to be left in peace to seek the truth 
in his own fashion. Little did he think that his 
expedition would be made the basis for a fabrica- 
tion of alleged astronomical discoveries destined 
to startle a hemisphere. Yet that is precisely 
what happened. Some time about the middle of 
the year 1835 the New York Sun began the pub- 
lication of certain articles, purporting to give an 
account of " Great Astronomical Discoveries, 
lately made by Sir John Herschel at the Cape of 
Good Hope." It was alleged that these articles 
were taken from a supplement to the Edinburgh 
Journal of Science; yet there is no doubt that 
they were manufactured entirely in the United 
States, and probably in New York. 

The hoax begins at once in a grandiloquent 
style, calculated to attract popular attention, and 
well fitted to the marvels about to be related. 
Here is an introductory remark, as a specimen : 
" It has been poetically said that the stars of 


heaven are the hereditary regalia of man as the 
intellectual sovereign of the animal creation. He 
may now fold the zodiac around him with a 
loftier consciousness of his mental supremacy." 
Then follows a circumstantial and highly plausible 
account of the manner in which early and exclu- 
sive information was obtained from the Cape. 
This was, of course, important in order to make 
people believe in the genuineness of the whole ; 
but we pass at once to the more interesting ac- 
count of Herschel's supposed instrument. 

Nothing could be more skilful than the way in 
which an air of truth is cast over the coming ac- 
count of marvellous discoveries by explaining in 
detail the construction of the imaginary Her- 
schelian instrument. Sir John is supposed to 
have had an interesting conversation in England 
"with Sir David Brewster, upon the merits of 
some ingenious suggestion by the latter, in his ar- 
ticle on optics in the Edinburgh Encyclopaedia 
(p. 644), for improvements in the Newtonian 
reflectors." The exact reference to a particular 
page is here quite delightful. After some further 
talk, " the conversation became directed to that 


all-invincible enemy, the paucity of light in pow- 
erful magnifiers. After a few moments' silent 
thought. Sir John diffidently inquired whether it 
would not be possible to effect a transfusion of 
artificial light through the focal object of vision ! 
Sir David, somewhat startled at the originality of 
the idea, paused awhile, and then hesitatingly re- 
ferred to the refrangibility of rays, and the an- 
gle of incidence. ... Sir John continued, 
' Why cannot the illuminated microscope, say 
the hydro-oxygen, be applied to render distinct, 
and, if necessary, even to magnify the focal ob- 
ject?' Sir David sprang from his chair in an 
ecstasy of conviction, and leaping half-way to the 
ceiling, exclaimed, ' Thou art the man.' " This 
absurd imaginary conversation contains nothing 
but an assemblage of optical jargon, put together 
without the slightest intention of conveying any 
intelligible meaning to scientific people. Yet it 
was well adapted to deceive the public ; and we 
should not be surprised if it would be credited 
by many newspaper readers to-day. 

The authors go on to explain how money was 

raised to build the new instrument, and then de- 



scribe Herschel's embarkation and the difficulties 
connected with transporting his gigantic ma- 
chines to the place selected for the observing 
station. " Sir John accomplished the ascent to 
the plains by means of two relief teams of oxen, 
of eighteen each, in about four days, and, aided 
by several companies of Dutch boors [«V], pro- 
ceeded at once to the erecting of his gigantic 
fabric." The place really selected by Herschel 
cannot be described better than in his own 
words, contained in a genuine letter dated Janu- 
ary 21, 1835: "A perfect paradise in rich and 
magnificent mountain scenery, sheltered from all 
winds. ... I must reserve for my next all 
description of the gorgeous display of flowers 
which adorn this splendid country, as well as 
the astonishing brilliancy of the constellations." 
The author of the hoax could have had. no 
knowledge of Herschel's real location, as de- 
scribed in this letter. 

The present writer can bear witness to the 
correctness of Herschel's words. Feldhausen 
is truly an ideal secluded spot for astronomical 

study. A small obelisk under the sheer cliff of 



far-famed Table Mountain now marks the site 
of the great reflecting telescope. Here Herschel 
carried on his scrutiny of the Southern skies. 
He observed 1,202 double stars and 1,708 
nebulae and clusters, of which only 439 were al- 
ready known. He studied the famous Magel- 
lanic clouds, and made the first careful drawings 
of the " keyhole " nebula in the constellation 

Very recent researches of the present royal 
astronomer at the Cape have shown that changes 
of import have certainly taken place in this 
nebula since Herschel's time, when a sudden 
blazing up of the wonderful star Eta Argus 
was seen within the nebula. This object has, 
perhaps, undergone more remarkable changes of 
light than any other star in the heavens. It is 
as though there were some vast conflagration at 
work, now blazing into incandescence, and again 
sinking almost into invisibility. In 1843 ^^- 
clear estimated the brilliancy of Eta to be about 
equal to that of Sirius, the brightest star in the 
whole sky. Later it diminished in light, and 

cannot be seen to-day with the naked eye, though 



the latest telescopic observations indicate that it 
is again beginning to brighten. 

Such was Herschel's quiet study of his beloved 
science, in glaring contrast to the supposed dis- 
coveries of the " Hoax." Here are a few things 
alleged to have been seen on the moon. The 
first time the instrument was turned upon our sat- 
ellite " the field of view was covered throughout 
its entire area with a beautifully distinct and even 
vivid representation of basaltic rock." There 
were forests, too, and water, " fairer shores never 
angels coasted on a tour of pleasure. A beach of 
brilliant white sand, girt with wild castellated 
rocks, apparently of green marble." 

There was animal life as well ; " we beheld 
continuous herds of brown quadrupeds, having 
all the external characteristics of the bison, 
but more diminutive than any species of the 
bos genus in our natural history." There was 
a kind of beaver, that " carries its young in 
its arms like a human being," and lives in huts. 
" From the appearance of smoke in nearly all of 
them, there is no doubt of its (the beaver's) be- 
ing acquainted with the use of fire." Finally, as 



was, of course, unavoidable, human creatures 
were discovered. "Whilst gazing in a perspec- 
tive of about half a mile, we were thrilled with 
astonishment to perceive four successive flocks of 
large-winged creatures, wholly unlike any kind 
of birds, descend with a slow, even motion from 
the cliffs on the western side, and alight upon 
the plain. . . . Certainly they were like hu- 
man beings, and their attitude in walking was 
both erect and dignified." 

We have not space to give more extended ex- 
tracts from the hoax, but we think the above 
specimens will show how deceptive the whole 
thing was. The rare reprint from which we have 
extracted our quotations contains also some inter- 
esting " Opinions of the American Press Respect- 
ing the Foregoing Discovery." The Daily Ad- 
vertiser said : " No article, we believe, has ap- 
peared for years, that will command so general a 
perusal and publication. Sir John has added a 
stock of knowledge to the present age that will 
immortalize his name and place it high on the 
page of science." The Mercantile Advertiser 

said : " Discoveries in the Moon. — We com- 



mence to-day the publication of an interesting 
article which is stated to have been copied from 
the Edinburgh Journal of Science, and which made 
its first appearance here in a contemporary journal 
of this city. It appears to carry intrinsic evidence 
of being an authentic document." Many other 
similar extracts are given. The New York Even- 
ing Post did not fall into the trap. The Evening 
Post's remarks were as follows : "It is quite 
proper that the Sun should be the means of shed- 
ding so much light on the Moon. That there 
should be winged people in the moon does not 
strike us as more wonderful than the existence of 
such a race of beings on the earth ; and that there 
does or did exist such a race rests on the evidence 
of that most veracious of voyagers and circum- 
stantial of chroniclers, Peter Wilkins, whose cele- 
brated work not only gives an account of the 
general appearance and habits of a most interest- 
ing tribe of flying Indians, but also of all those 
more delicate and engaging traits which the author 
was enabled to discover by reason of the conjugal 
relations he entered into with one of the females 

of the winged tribe." 



We shall limit our extracts from the contem- 
porary press to the few quotations here given, 
hoping that enough has been said to direct atten- 
tion once more to that important subject, the 
Possibility of Being Deceived. 



Three generations of men have come and 
gone since the Marquis de Laplace stood before 
the Academy of France and gave his demonstra- 
tion of the permanent stability of our solar sys- 
tem. There was one significant fault in New- 
ton's superbly simple conception of an eternal 
law governing the world in which we live. The 
labors of mathematicians following him had 
shown that the planets must trace out paths in 
space whose form could be determined in ad- 
vance with unerring certainty by the aid of 
Newton's law of gravitation. But they proved 
just as conclusively that these planetary orbits, 
as they are called, could not maintain indefinitely 
the same shapes or positions. Slow indeed might 
be the changes they were destined to undergo ; 
slow, but sure, with that sureness belonging to 
celestial science alone. And so men asked : 
Has this magnificent solar system been built 


upon a scale so grand, been put in operation sub- 
ject to a law sublime in its very simplicity, only 
to change and change until at length it shall lose 
every semblance of its former self, and end, per- 
haps, in chaos or extinction ? 

Laplace was able to answer confidently, " No." 
Nor was his answer couched in the enthusiastic 
language of unbalanced theorists who work by 
the aid of Imagination alone. Based upon the 
irrefragable logic of correct mathematical reason- 
ing, and clad in the sober garb of mathematical 
formulas, his results carried conviction to men of 
science the world over. So was it demonstrated 
that changes in our solar system are surely at 
work, and shall continue for nearly countless 
ages ; yet just as surely will they be reversed at 
last, and the system will tend to return again to 
its original form and condition. The objection 
that the Newtonian law meant ultimate dissolu- 
tion of the world was thus destroyed by Laplace. 
From that day forward the law of gravitation 
has been accepted as holding sway over all phe- 
nomena visible within our planetary world. 

The intricacies of our own solar system being 


thus illumined, the restless activity of the human 
intellect was stimulated to search beyond for new 
problems and new mysteries. Even more fas- 
cinating than the movements of our sun and 
planets are all those questions that relate to the 
clustered stellar congeries hanging suspended 
within the deep vault of night. Does the same 
law of gravitation cast its magic spell over that 
hazy cloud of Pleiades, binding them, like our- 
selves, with bonds indissoluble ? Who shall an- 
swer, yes or no ? We can only say that astrono- 
mers have as yet but stepped upon the threshold 
of the universe, and fixed the telescope's great 
eye upon that which is within. 

Let us then begin by reminding the reader 
what is meant by the Newtonian law of gravita- 
tion. It appears all things possess the remark- 
able property of attracting or pulling each other. 
Newton declared that all substances, solid, liq- 
uid, or even gaseous — from the massive cliff of 
rock down to the invisible air — all matter can 
no more help pulling than it can help existing 
His law further formulates certain conditions 
governing the manner in which this gravitational 


attraction is exerted ; but these are mere matters 
of detail ; interest centres about the mysterious 
fact of attraction itself. How can one thing pull 
another with no connecting link through which 
the pull can act ? Just here we touch the point 
that has never yet been explained. Nature with- 
holds from science her ultimate secrets. They 
that have pondered longest, that have descended 
farthest of all men into the clear well of knowl- 
edge, have done so but to sound the depths be- 
yond, never touching bottom. 

This inability of ours, to give a good physical 
explanation of gravitation, has led certain makers 
of paradoxes to doubt or even deny that there is 
any such thing. But, fortunately, we have a sim- 
ple laboratory experiment that helps us. Un- 
explained it may ever remain, but that there 
can be attraction between physical objects con- 
nected by no visible link is proved by the be- 
havior of an ordinary magnet. Place a small 
piece of steel or iron near a magnetized bar, and 
it will at once be so strongly attracted that it 
will actually fly to the magnet. Anyone who 
has seen this simple experiment can never again 



deny the possibility, at least, of the law of attrac- 
tion as stated by Newton. Its possibility once 
admitted, the fact that it can predict the motions 
of all the planets, even down to their minutest 
details, transforms the possibility of its truth into 
a certainty as strong as any human certainty can 
ever be. 

But this demonstration of Newton's law is 
limited strictly to the solar system itself. We 
may, indeed, reason by analogy, and take for 
granted that a law which holds within our imme- 
diate neighborhood is extremely likely to be true 
also of the entire visible universe. But men of 
science are loath to reason thus ; and hence the 
fascination of researches in cosmic astronomy. 
Analogy points out the path. The astronomer 
is not slow to follow ; but he seeks ever to 
establish upon incontrovertible evidence those 
truths which at first only his daring imagination 
had led him to half suspect. 

If we are to extend the law of gravitation to 
the utmost, we must be careful to consider the 
law Itself in its most complete form. A heavenly 
body like the sun is often said to govern the 



motions of its family of planets ; but such a state- 
ment is not strictly accurate. The governing 
body is no despot ; 'tis an abject slave of law and 
order, as much as the tiniest of attendant planets. 
The action of gravitation is mutual, and no 
cosmic body can attract another without being 
itself in turn subject to that other's gravitational 

If there were in our solar system but two bodies, 
sun and planet, we should find each one pursu- 
ing a path in space under the influence of the 
other's attraction. These two paths or orbits 
would be oval, and if the sun and planet were 
equally massive, the orbits would be exactly 
alike, both in shape and size. But if the sun 
were far larger than the planet, the orbits would 
still be similar in form, but the one traversed by 
the larger body would be small. For it is not 
reasonable to expect a little planet to keep the 
big sun moving with a velocity as great as that 
derived by itself from the attraction of the 
larger orb. 

Whenever the preponderance of the larger 
body is extremely great, its orbit will be corre- 



spondingly insignificant in size. This is in fact 
the case with our own sun. So massive is it in 
comparison with the planets that the orbit is too 
small to reveal its actual existence without the 
aid of our most refined instruments. The path 
traced out by the sun's centre would not fill a 
space as large as the sun's own bulk. Neverthe- 
less, true orbital motion is there. 

So we may conclude that as a necessary con- 
sequence of the law of gravitation every object 
within the solar system is in motion. To say 
that planets revolve about the sun is to neglect 
as unimportant the small orbit of the sun itself. 
This may be sufficiently accurate for ordinary 
purposes ; but it is unquestionably necessary to 
neglect no factor, however small, if we propose 
to extend our reasoning to a consideration of the 
stellar universe. For we shall then have to deal 
with systems in which the planets are of a size 
comparable with the sun ; and in such systems 
all the orbits will also be of comparatively equal 

Mathematical analysis has derived another feet 
from discussion of the law of gravitation which, 



perhaps, transcends in simple grandeur every- 
thing we have as yet mentioned. It matters not 
how great may be the number of massive orbs 
threading their countless interlacing curved paths 
in space, there yet must be in every cosmic sys- 
tem one single point immovable. This point is 
called the Centre of Gravity. If it should so 
happen that in the beginning of things, some 
particle of matter were situated at this centre, 
then would that atom ever remain unmoved and 
imperturbable throughout all the successive vicis- 
situdes of cosmic evolution. It is doubtful 
whether the mind of man can form a conception 
of anything grander than such an immovable 
atom within the mysterious intricacies of cosmic 

But in general, we cannot suppose that the 
centres of gravity in the various stellar systems 
are really occupied by actual physical bodies. 
The centre may be a mere mathematical point in 
space, situated among the several bodies compos- 
ing the system, but, nevertheless, endowed, in a 
certain sense, with the same remarkable property 

of relative immobility. 



Having thus defined the centre of gravity in 
its relation to the constituent parts of any cosmic 
system, we can pass easily to its characteristic 
properties in connection with the inter-relation 
of stellar systems with one another. It can be 
proved mathematically that our solar system will 
pull upon distant stars just as though the sun 
and all the planets were concentrated into one 
vast sphere having its centre in the centre of 
gravity of the whole. It is this property of the 
centre of gravity which makes it pre-eminently 
important in cosmic researches. For, while we 
know that centre to be at rest relatively to all 
the planets in the system, it may, nevertheless, 
in its quality as a sort of concentrated essence of 
them all, be moving swiftly through space under 
the pull of distant stars. In that case, the at- 
tendant bodies will go with it — but they will 
pursue their evolutions within the system, all un- 
conscious that the centre of gravity is carrying 
them on a far wider circuit. 

What is the nature of that circuit? This 
question has been for many years the subject of 

earnest study by the clearest minds among as- 



tronomers. The greatest difficulty in the way is 
the comparatively brief period during which men 
have been able to make astronomical observa- 
tions of precision. Space and time are two con- 
ceptions that transcend the powers of definition 
possessed by any man. But we can at least 
form a notion of how vast is the extent of time, 
if we remember that the period covered by man's 
written records is registered but as a single mo- 
ment upon the great revolving dial of heaven's 
dome. One hundred and fifty years have 
elapsed since James Bradley built the founda- 
tions of modern sidereal astronomy upon his mas- 
terly series of observations at the Royal Observa- 
tory of Greenwich, in England. Yet so slowly 
do the movements of the stars unroll themselves 
upon the firmament, that even to this day no 
one of them has been seen by men to trace out 
more than an infinitesimal fraction of its destined 
path through the voids of space. 

Travellers upon a railroad cannot tell at any 
given moment whether they are moving in a 
straight line, or whether the train is turning 

upon some curve of huge size. The St. Goth- 



ard railway has several so-called " corkscrew " 
tunnels, within which the rails make a complete 
turn in a spiral, the train finally emerging from 
the tunnel at a point almost vertically over the 
entrance. In this way the train is lifted to a 
higher level. Passengers are wont to amuse 
themselves while in these tunnels by watching 
the needle of an ordinary pocket-compass. This 
needle, of course, always points to the north ; 
and as the train turns upon its curve, the needle 
will make a complete revolution. But the pas- 
senger could not know without the compass 
that the train was not moving in a perfectiy 
straight line. Just so we passengers on the 
earth are unaware of the kind of path we are 
traversing, until, like the compass, the astron- 
omer's instruments shall reveal to us the 

But as we have seen, astronomical observations 
of precision have not as yet extended through a 
period of time corresponding to the few minutes 
during which the St. Gothard traveller watches 
the compass. We are still in the dark, and do 
not know as yet whether mankind shall last long 


enough upon the earth to see the compass needle 
make its revolution. We are compelled to be- 
lieve that the motion in space of our sun is pro- 
gressing upon a curved path ; but so far as pre- 
cise observations allow us to speak, we can but 
say that we have as yet moved through an infini- 
tesimal element only of that mighty curve. How- 
ever, we know the point upon the sky toward 
which this tiny element of our path is directed, 
and we have an approximate knowledge of the 
speed at which we move. 

More than a century ago Sir William Herschel 
was able to fix roughly what we call the apex of 
the sun's way in space, or the point among the 
stars toward which that way is for the moment 
directed. We say for the moment, but we mean 
that moment of which Bradley saw the beginning 
in 1750, and upon whose end no man of those 
now living shall ever look. Herschel found that 
a comparison of old stellar observations seemed 
to indicate that the stars in a certain part of the 
sky were opening out, as it were, and that the 
constellations in the opposite part of the heavens 
seemed to be drawing in, or becoming smaller. 


There can be but one reasonable explanation of 
this. We must be moving toward that part of 
the sky where the stars are separating. Just so a 
man watching a regiment of soldiers approaching, 
will see at first only a confused body of men ; 
but as they come nearer, the individual soldiers 
will seem to separate, until at length each one is 
seen distinct from all the others. 

Herschel fixed the position of the apex at a 
point in the constellation Hercules. The most 
recent investigations of Newcomb and others 
have, on the whole, verified Herschel's conclu- 
sions. With the intuitive power of rare genius, 
Herschel had been able to sift truth out of error. 
The observational data at his disposal would now 
be called rude, but they disclosed to the scrutiny 
of his acute understanding the germ of truth that 
was in them. Later investigators have increased 
the precision of our knowledge, until we can now 
say that the present direction of the solar motion 
is known within very narrow limits. A tiny circle 
might be drawn on the sky, to which an astrono- 
mer might point his hand and say : " Yonder little 
circle contains the goal toward which the sun and 


planets are hastening to-day." Even the speed 
of this motion has been subjected to measure- 
ment, and found to be about ten miles per 

The objective point and the rate of motion 
thus stated, exact science holds her peace. Here 
genuine knowledge stops ; and we can proceed 
further only by the aid of that imagination which 
men of science need to curb at every moment. 
But let no one think that the sun will ever 
reach the so-called apex. To do so would mean 
cosmic motion upon a straight line, while every 
consideration of celestial mechanics points to mo- 
tion upon a curve. When shall we turn suffi- 
ciently upon that curve to detect its bending? 
'Tis a problem we must leave as a rich heritage 
to later generations that are to follow us. The 
visionary theorist's notion of a great central sun, 
controlling our own sun's way in space, must be 
dismissed as far too daring. But for such a cen- 
tral sun we may substitute a central centre of 
gravity belonging to a great system of which our 
sun is but an insignificant member. Then we 
reach a conception that has lost nothing in the 



grandeur of its simplicity, and is yet in accord 
with the probabilities of sober mechanical science. 
We cease to be a lonely world, and stretch out 
the bonds of a common relationship to yonder 
stars within the firmament. 




Airy, Astronomer Royal i 

AUis, photographs comet loi 

Andromeda nebula 28 

temporary star 28, 29, 45 

Apex, of solar motion, explained 221 

Aquila, constellation, temporary star in 40 

Arctic regions, position of pole in I94 

Argo, constellation, variable star in 205 

Association, international geodetic 139 

Asteroids, first discovery by Piazzi 59> 1°^ 

discovery by photography 64 

group of 63 

photography of, invented by Wolf 104 

Astronomer, royal i 

working, description of 152 

Astronomer's Pole, the 184 

Astronomy, journalistic 17^ 

practical uses of 112 

Atmospheric refraction, explained 193 

Axis, of figure of the earth 136 

of rotation of the earth 136 

polar, of telescope 173 

Barnard, discovers satellite of Jupiter 51 

Bessel, measures Pleiades 15 

Bond, discovers crape ring of Saturn 144 

Bradley, observes at Greenwich 219 

Brahe, Tycho, his temporary star 40 

Bruce, endows polar photography 197 




Campbell, observes Pole-star i8 

Cape of Good Hope, observatory, photography at loi 

telescope 1 70, 1 74 

Capriccio, Galileo's 55 

Cassini, shows Saturn's rings to be double I44 

Cassiopeia, temporary star in 4° 

Celestial pole 184 

Central sun theory 223 

Centre of gravity 217 

Chart-room, on ship-board 5 

Chronometer, invention of 8 

Circle, meridian, explained 189 

Clerk- Maxwell, discusses Saturn's rings 146 

Clock, affected by temperature 117 

affected by barometric pressure 117 

astronomical n 5 

astronomical, how mounted 116 

astronomical, its dial 116 

error of, determined with transit 118 

jeweller's regulator 114 

of telescope I75 

Clusters of stars, photography of 98 

Columbia University Observatory, latitude observations 139 

polar photography 19^ 

Common, his reflecting telescope 32 

Confusion of dates, in Pacific Ocean 125 

Congress of Astronomers, Paris, 1887 102 

Constellations 162 

Control, "mouse," for photography 88 

Copemican theory of universe 53>5^ 

demonstration 94 

Corkscrew tunnels 220 

Crape ring of Saturn 144 

Cumulative effect, in photography 84 

Date, confusion of, in Pacific Ocean 125 

Date-line, international, explained 126 

Development of photograph 81 




Dial, of astronomical clock Il6 

"Dialogue " of Galileo 53 

Differences of time, explained 121 

Directions, telescopic measurement of 21 

Directory of the heavens 103 

Distance, of light-source in photography 83 

of stars 94, 106, 158 

of Sun 67, 97, 106 

Donner, polar photography 195 

Double telescopes, for photography 86 

Earth, motions of its pole 131 

rotation of 136, 162, 171, 184 

shape of 135 

Eclipses, photography of 109 

Elkin, measures Pleiades 15 

Equatorial telescope, explained 1 70 

Eros, discovered by Witt 66, 105 

its importance 67 

Error of clock, determined by transit 118 

Exposure, length of, in photography 84 

Feldhausen, Herschel's observatory near Capetown 204 

Fiji Islands, their date 126 

Fixed polar telescope 197 

" Following " the stars 88, 173 

Four-day cycle of pole-star 24 

France, outside time-zone system 129 

Fundamental longitude meridian 124 

Galileo 47 

and the Church 48 

discoveries of 49 

observes Saturn 141 

Galle, discovers Neptune 61 

Gauss, computes first asteroid orbit 60 

Gautier, Paris, constructs big telescope 179 

Geodetic Association, international , 139 




Geography, maps, astronomical side of 112 

Geology, polar motion in 131 

Gill, photographs comet 100 

Gilliss, at Naval Observatory, Washington 169 

Goldsborough, at Naval Observatory, Washington 169 

Grande Lunette, Paris, 1900 176, 180 

Gravitation 13 

in Pleiades 14, 212 

law of, Newton's 212 

Gravity, centre of 217 

Greenwich, origin of longitudes 7, 124 

time 7 

Groombridge, English astronomer i 

Harrison, inventor of chronometer 8 

Head, of heliometer 156 

Heidelberg, photography at 104 

Heliometer 152 

head of 156 

how used 157 

principle of 154 

scales of 158 

semi-lenses of 155 

Helsingfors observatory, polar photography at 195 

Henry, measures Pleiades 11, 17 

Hercules, constellation, solar motion toward 222 

Herschel, discovers apex of solar motion 221 

discovers Uranus 59, 141 

John, the moon hoax 200 

Hipparchus, discovers precession 186 

early star catalogue 21, 39 

invents star magnitudes 91 

Huygens, announces rings of Saturn 142 

his logogriph 143 

Ice-cap, of Earth ; 131 

Index Librorum Prohibitorum 53 

International, date-line, explained 126 

geodetic association 139 




Inter-Stellar motion, in clusters. . , 98 

in Pleiades 14 

Islands of Pacific, their longitude and time 125 

Japan, latitude station in 139 

Jewellers' correct time 121 

Journalistic astronomy 176 

Jupiter's satellites, discovered by Galileo 50 

discovered by Barnard 51 

Keeler, observes Saturn's rings 140, 147, 150 

photographs nebulse 32 

" Keyhole " nebula 205 

Lambert, determines longitude of Washington 168 

Laplace, discusses Saturn's rings 146 

nebular hypothesis 33 

stability of solar system 210 

Latitude, changes of 133, 138 

definition of 134 

determining the 6 

Leverrier, predicts discovery of Neptune 61, 142 

Lick Observatory, Keeler's observations 140 

Light, undulatory theory of 19, 148 

Light-waves, measuring length of 20, 149 

Logogriph, by Huygens 143 

Long-exposure photography 85 

Longitude, counted East and West 125 

determining 6 

determining by occultations 167 

effect on time differences 123 

explained 123 

of Washington, first determined 168 

Maclear, observes Eta Argus 205 

Magnitudes, stellar 91 

Manila, its time 127 

Maps, astronomical side of 112 




Meridian circle, explained 189 

Milky- way, poor in nebulse 33 

Minor Planets, see Asteroids. 

Moon, Hoax igg 

motion among stars 163 

mountains discovered by Galileo 49 

size of, measured 166 

Motion of moon 163 

Motions of the Earth's Pole 131 

Mounting Great Telescopes 170 

Naked-eye nebulae 28 

Naples, Royal Observatory, latitude observations 139 

Naval Observatory, Washington, noon signal 120 

Navigation i 

before chronometers 3 

use of astronomy in 113 

NebuL/E 27 

Nebula, in Andromeda 28 

in Orion 30 

" keyhole " 205 

Nebular, hypothesis 33 

structure in Pleiades 17 

Nebulous stars 31 

Negative, and positive, in photography 82 

Neptune, discovery predicted by Leverrier 61, 142 

discovery by Galle 61 

Newcomb, fixes apex of solar motion 222 

Newton, law of gravitation 212 

longitude commission 8 

New York, its telegraphic time system 120 

Noon Signal, Washington 120 

Number, of nebulae 31, 33 

of temporary stars 38 

Nutation, explained 188 

Occultations 161 

explained 165 




Occultations, use of 166, 167 

Orion nebula 30 

Pacific islands, their longitude and time 125 

Parallax, solar 67, 106 

stellar 94, 106 

measured with heliometer 158 

Paris, congress of astronomers, 1887 102 

exposition of igoo 176 

Periodic motion of earth's pole 133 

Perseus, constellation, temporary star in 46 

Philippine Islands, their time 127 

Photography, asteroid, invented by Wolf 104 

congress of astronomical 102 

cumulative effect of light 84 

distance of light-source 83 

double telescopes for 86 

general star catalogue 102 


in discovery of asteroids 64, 104 

in solar physics 109 

in spectroscopy 108 

length of exposure 84 

measuring machine, Rutherfurd 93 

motion of telescope for 87 

" mouse " control of telescope 88 

of eclipses 109 

of inter-stellar motion 99 

Paris congress, 1877 102 

polar 191 

Rutherfurd pioneer in 90 

star-clusters 98 

star-distances measured by 94 

summarized no 

wholesale methods in 103 

Piazzi, discovers first asteroid 59, 106 

Pitkin, report to House of Representatives 168 

Planetary nebulse 31 




Planet of 1898 58 

Planetoids, see Asteroids. 

Planets known to ancients 58 

Pleiades 10 

gravitation among 212 

motion among 14, 16, 98 

nebular structure 17 

number visible 11 

Polar axis, of telescope 173 

Polar photography 191 

at Helsingfors 195 

Pole, celestial 184 

of the earth, motions of 131 

THE Astronomer's 184 

Pole-Star 18 

as a binary , 25 

as a triple 18, 26 

change of 187 

its four-day cycle 24 

motion toward us 24 

Positive, and negative, in photography 82 

Potsdam, observatory, photographic star-catalogue 103 

Practical uses of astronomy 112 

Precession, explained 186 

Prize, for invention of chronometer 8 

Ptolemaic theory of universe S^ 

Ptolemy, writes concerning Hipparchus 39 

Railroad time, explained 127 

Refraction, atmospheric, explained 193 

" Regulator," the jeweller's clock 114 

Ring-nebulae 31 

Rings, of Saturn, see Saturn's rings. 

Roberts, Andromeda nebula 28 

Rotation, of Earth 136, 162, 171, 184 

of Saturn 150 

Royal Astronomer, his duties 2 

Royal Observatory, Greenwich 1 24 




Greenwich, Bradley's observations 219 

Naples, latitude observations 1 39 

Rutherfurd, cluster photography 99 

invents photographic apparatus 93 

pioneer in photography 90 

stellar parallax 94 

Sagredus, character in Galileo's Dialogue 55 

Salusbury, Galileo's translator 5°! 54 

Salviati, character in Galileo's Dialogue 55 

Samoa, its date 126 

Saturn's Rings 140 

analogy to planetoids 147 

announced by Huygens 142 

observed with spectroscope 147 

shown to be double by Cassini 144 

structure and stability 145 

Scales, of heliometer 158 

Scorpio, constellation, temporary star in 39 

Semi-lenses of heliometer 155 

Sextant, how used 4 

Sicily, latitude station in 1 39 

Sidereus Nuncius, published by Galileo 52 

Simplicio, character in Galileo's Dialogue 55 

Sirius, brightest star 205 

Size of Moon, measured 166 

SociHi de VOptique 177 

Solar parallax, see Sun's distance. 

physics, by photography 109 

system, stability of 210 

Spectroscope, its use explained 147 

used on pole-star 19 

to observe Saturn's rings 147 

Spiral nebulae 31 

Stability, of Saturn's nngs 145 

of Solar System 210 

Standards, time, of the world u i 

table of 130 




' ' Standard " time, explained 1 27 

Star-catalogue, general photographic 102 

Star-clusters, photography of 98 

Star-distances 94, io5 

measured with heliometer 1 58 

Rutherfurd 94 

Star magnitudes 91 

Star-motion, toward us 21 

Star-tables, astronomical 118 

Stars, variable 42 

St. Gothard railway, tunnels 220 

Sun, newspaper, the moon hoax 201 

SuN-DiAL, How TO Make a 69 

Sun's, Destination 210 

distance, compared with star distance 97 

measured with Eros 67, 106 

motion, apex of 221 

Sun-spots, discovered by Galileo 49 

Sy sterna Satumium, Huygens 143 

Telescope, clock i75 

at Paris Exposition 176, 180 

double, for photography 86 

equatorial, explained 17'' 

first used by Galileo 49 

motion of 87 

mounting great I?" 

unmoving, for polar photography 197 

Temporary Stars 37 

in Andromeda nebula 28, 29, 45 

in Aquila 40 

in Cassiopeia 40 

in Perseus 46 

in Scorpio 39 

their number 3^ 

theory of 42 

Time, correct, determined astronomically 113 

differences between different places 121 




Time Standards of the World 1 1 1 

standards of the World, table of 130 

system, in New York 120 

zones, explained 128 

Trails, photographic 191 

Transit, for determining clock error I18 

Tycho Brahe, his temporary star 40 

Ulugh Beg, early star catalogue 21 

Undulatory theory, of light 19, 148 

Universe, theories of 34, 53, 56 

Uranus, discovered by Herschel 59, 142 

Use of occultations i6fa, 1 67 

Uses of astronomy, practical 112 

Variable stars 42 

in Argo 205 

Vega, future pole-star 187 

Visibility of stars, in day-time 191 

Vision, phenomenon of 20, 149 

Washington, its longitude first determined 168 

Waves, explained 148 

of light 20, 148 

Wilkes, at Naval Observatory, Washington 169 

Wilkins, imaginary voyage of 208 

Witt, discovers Eros 66, 105 

Wolf, M., invents asteroid photography 104 

measures Pleiades 11 

World's time standards, table of 130 

Yale College, Pleiades measured at 15 

Zones, time, explained 128